Modality: Syntactic reference¶
Must¶
Minimal contrasts¶
He must leave. (obligation; plain infinitival)
He must be leaving. (present ongoing inference; progressive infinitival)
He must have left. (past inference; perfect infinitival)
He had to leave. (past obligation; finite clause; not a modal construction)
Licensed constructions¶
He must leave. (obligation; plain infinitival)
He must leave tomorrow. (future obligation; plain infinitival)
He must be at home. (present inference; plain infinitival)
He must know the answer. (present inference; plain infinitival)
He must be sleeping. (present ongoing inference; progressive infinitival)
He must be working. (present ongoing inference; progressive infinitival)
He must be arrested. (obligation; passive infinitival)
He must have left. (past inference; perfect infinitival)
He must have seen us. (past inference; perfect infinitival)
He must have been sleeping. (past ongoing inference; perfect progressive infinitival)
He must have been working all day. (past ongoing inference; perfect progressive infinitival)
He must have been arrested. (past inference; perfect passive infinitival)
The report must have been completed already. (past inference; perfect passive infinitival)
You mustn’t leave. (prohibition; plain infinitival)
You mustn’t tell anyone. (prohibition; plain infinitival)
Ungrammatical¶
He must to leave.(plain infinitive required)
He must left.(plain form required)
He must have saw it.(past participle required)
He must had left.(perfect infinitive required)
He must have had seen us.(past participle required)
He must leaving.(plain form required)
To must leave is difficult.(no non-finite forms)
Technical notes¶
No preterite form.
No non-finite forms.
No to-infinitive complement.
Past obligation normally uses had to, not must.
Mustn’t expresses prohibition, not lack of necessity.
Negative epistemic “must not” is possible but rare; it expresses a strong negative inference:
He must not know the answer. (negative inference)
Interpretation clues¶
Obligation: the subject is required or expected to perform the action.
You must leave now.
Inference: the speaker draws a conclusion from available evidence.
He must be at home.
The infinitive construction locates the inferred situation:
plain infinitive → present state or situation
He must know the answer.
progressive infinitive → ongoing situation
He must be sleeping.
perfect infinitive → past event or state
He must have left.
perfect progressive infinitive → ongoing past situation
He must have been sleeping.
perfect passive infinitive → past passive situation
He must have been arrested.
Prohibition: expressed with mustn’t.
You mustn’t park here.
Can¶
Minimal contrasts¶
He can swim. (ability; plain infinitival)
He can be swimming. (present ongoing possibility; progressive infinitival)
He can have swum. (past possibility; perfect infinitival)
He could swim. (past/general ability; preterite modal)
He was able to swim. (specific successful past ability)
Licensed constructions¶
He can swim. (ability; plain infinitival)
He can solve the problem. (ability; plain infinitival)
He can leave now. (permission; plain infinitival)
He can be at home. (present possibility; plain infinitival)
He can be sleeping. (present ongoing possibility; progressive infinitival)
He can be seen from the road. (possibility; passive infinitival)
He can have left already. (past possibility; perfect infinitival)
He can have forgotten. (past possibility; perfect infinitival)
He can have been sleeping. (past ongoing possibility; perfect progressive infinitival)
He can have been arrested. (past possibility; perfect passive infinitival clause)
You can’t enter. (prohibition; plain infinitival)
You can’t be serious. (present impossibility; plain infinitival)
He can’t have done it. (past impossibility; perfect infinitival)
The door can’t have been locked. (past impossibility; perfect passive infinitival)
Ungrammatical¶
He can to swim. (plain form required)
He can swims. (plain form required)
He can swimming. (plain form required)
He can swam. (plain form required)
He has can swim. (no participle form)
He is can swim. (no non-finite form)
To can swim is useful. (no infinitive form)
Technical notes¶
No infinitive form (
to canis not possible).No participial forms as a modal (
canning,cannedare not modal forms).No third-person singular -s (he
cansis not modal can).Past ability may require was/were able to for a specific successful event:
She was able to escape.
Negative can’t has two distinct uses:
He can’t swim. (inability)
He can’t have left already. (impossibility)
Interpretation clues¶
Ability: the subject has the skill, capacity, or physical possibility to perform the action.
She can swim.
Permission: the subject is allowed to perform the action.
You can leave now.
Possibility: the speaker presents a situation as possible.
It can happen.
Inference / impossibility (can’t): the speaker rejects a conclusion as impossible.
He can’t have done it.
The infinitive construction locates the possible or impossible situation:
plain infinitive → present state or situation
He can be at home.
progressive infinitive → ongoing situation
He can be sleeping.
perfect infinitive → past event or state
He can have left.
perfect progressive infinitive → ongoing past situation
He can have been sleeping.
perfect passive infinitive → past passive situation
He can have been arrested.
Negative can’t:
inability
He can’t swim.
impossibility
He can’t have left already.
Note
Permission, ability, and possibility are separate here because, unlike “must”, the same basic can + plain infinitive construction can represent several different meanings.
The context clue is usually the relationship between the subject and the action:
Subject’s capacity: ability
Authority/rules: permission
Speaker’s judgement: possibility
Could¶
Minimal contrasts¶
He could swim. (past general ability; plain infinitival)
He could be swimming. (present ongoing possibility; progressive infinitival)
He could have swum. (past possibility; perfect infinitival)
He was able to swim. (specific successful past ability)
Licensed constructions¶
He could swim. (past general ability; plain infinitival)
He could solve the problem. (past general ability; plain infinitival)
He could leave early. (permission; plain infinitival)
He could be at home. (present possibility; plain infinitival)
He could be sleeping. (present ongoing possibility; progressive infinitival)
He could be seen from here. (possibility; passive infinitival)
He could have left already. (past possibility; perfect infinitival)
He could have forgotten. (past possibility; perfect infinitival)
He could have been sleeping. (past ongoing possibility; perfect progressive infinitival)
He could have been arrested. (past possibility; perfect passive infinitival)
You couldn’t enter. (lack of ability/permission; plain infinitival)
He couldn’t be serious. (present impossibility; plain infinitival)
He couldn’t have done it. (past impossibility; perfect infinitival)
The door couldn’t have been locked. (past impossibility; perfect passive infinitival)
Ungrammatical¶
He could to swim.(plain form required
He could swims.(plain form required)
He could swimming.(plain form required)
He could swam.(plain form required)
He has could swim.(no participle form)
He is could swim.(no non-finite form)
To could swim is useful.(no infinitive form)
Technical notes¶
Preterite form of can.
No infinitive form (
to couldis not possible).No participial forms as a modal (
coulding,couldedare not modal forms).No third-person singular -s (he
couldsis not possible).Expresses past general ability, not normally a specific successful past event.
Frequently marks modal remoteness (tentativeness, politeness, hypotheticality) rather than past time.
Could have + past participle expresses past possibility, not past ability. For a specific successful event, was/were able to is normally preferred.
Interpretation clues¶
Past ability: the subject had the capacity to perform an action in the past.
She could swim when she was young.
Remote possibility: the speaker presents a possibility with distance, uncertainty, or tentativeness.
He could be at home.
Polite request / suggestion: often used in questions to make a request less direct.
Could you help me?
Counterfactual possibility: the speaker refers to a possible action or outcome that did not happen or is uncertain.
You could have told me.
The infinitive construction helps locate the possibility:
plain infinitive → present or general situation
He could be right.
progressive infinitive → ongoing situation
He could be sleeping.
perfect infinitive → past possibility
He could have left.
perfect progressive infinitive → ongoing past possibility
He could have been sleeping.
perfect passive infinitive → past passive possibility
He could have been arrested.
Specific successful past ability: usually avoid could and use was/were able to.
She was able to escape.
Note
Could + plain infinitive can mean either:
She could swim. (past ability)
He could be at home. (present remote possibility)
May¶
Minimal contrasts¶
He may leave now. (permission; plain infinitival)
He may be sleeping. (present ongoing possibility; progressive infinitival)
He may have left. (past possibility; perfect infinitival)
He may have been sleeping. (past ongoing possibility; perfect progressive infinitival)
Licensed constructions¶
He may leave now. (permission; plain infinitival)
He may take another copy. (permission; plain infinitival)
He may be at home. (present possibility; plain infinitival)
He may know the answer. (present possibility; plain infinitival)
He may be sleeping. (present ongoing possibility; progressive infinitival)
He may be seen from the road. (possibility; passive infinitival)
He may have left already. (past possibility; perfect infinitival)
He may have forgotten. (past possibility; perfect infinitival)
He may have been sleeping. (past ongoing possibility; perfect progressive infinitival)
He may have been arrested. (past possibility; perfect passive infinitival)
You may not leave yet. (permission withheld; plain infinitival)
He may not be at home. (present possibility ,negative; plain infinitival)
He may not have seen us. (past possibility (negative); perfect infinitival)
Ungrammatical¶
He may to leave.(plain form required)
He may leaves.(plain form required)
He may leaving.(plain form required)
He may left.(plain form required)
He has may leave.(no participle form)
He is may leave.(no non-finite form)
To may leave is permitted.(no infinitive form)
Technical notes¶
No preterite form.
No infinitive form (
to mayis not possible).No participial forms as a modal (
maying,mayedare not modal forms).No third-person singular -s (he
maysis not possible).Used for both permission and epistemic possibility.
May not is ambiguous.
Permission withheld:
You may not leave yet.
Negative possibility:
He may not be at home.
Interpretation clues¶
Permission: the speaker gives formal or polite permission.
You may leave now.
Possibility: the speaker presents a situation as possible.
He may be at home.
Negative may not: distinguish between:
permission withheld
You may not enter.
negative possibility
He may not know the answer.
The infinitive construction locates the possible situation:
plain infinitive → present state or situation
He may know the answer.
progressive infinitive → ongoing situation
He may be sleeping.
perfect infinitive → past event or state
He may have left.
perfect progressive infinitive → ongoing past situation
He may have been sleeping.
perfect passive infinitive → past passive situation
He may have been arrested.
Note
Formal style clue: may is more common than can for granting permission in formal contexts.
You may begin the exam.
You can begin the exam. (more informal)
Might¶
Minimal contrasts¶
He might leave. (remote possibility; plain infinitive)
He might be leaving. (present ongoing remote possibility; progressive infinitive)
He might have left. (past remote possibility; perfect infinitive)
He might have been leaving. (past ongoing remote possibility; perfect progressive infinitive)
Licensed constructions¶
He might leave now. (remote possibility; plain infinitive)
He might be at home. (present remote possibility; plain infinitive)
He might know the answer. (present remote possibility; plain infinitive)
He might be sleeping. (present ongoing remote possibility; progressive infinitive)
He might be seen from the road. (remote possibility; passive infinitive)
He might have left already. (past remote possibility; perfect infinitive)
He might have forgotten. (past remote possibility; perfect infinitive)
He might have been sleeping. (past ongoing remote possibility; perfect progressive infinitive)
He might have been arrested. (past remote possibility; perfect passive infinitive)
You might try again. (tentative suggestion; plain infinitive)
It might not work. (negative possibility; plain infinitive)
He might not have seen us. (negative past possibility; perfect infinitive)
Ungrammatical¶
He might to leave.(plain form required)
He might leaves.(plain form required)
He might leaving.(plain form required)
He might left.(plain form required)
He has might leave.(no participle form)
He is might leave.(no non-finite form)
To might leave is possible. (no infinitive form)
Technical notes¶
Historically the preterite form of “may”.
No infinitive form (
to mightis not possible).No participial forms as a modal (
mighting,mightedare not modal forms).No third-person singular -s (he
mightsis not possible).Commonly expresses modal remoteness rather than past time.
Generally presents a weaker or more remote possibility than may.
Perfect might have + past participle expresses a past remote possibility:
He might have forgotten.
Interpretation clues¶
Remote possibility: the speaker presents a possibility with greater uncertainty or distance.
He might be at home.
Tentative suggestion: often used to make a suggestion less direct.
You might try restarting the computer.
Negative possibility: the speaker considers a situation unlikely but possible.
He might not know the answer.
The infinitive construction locates the possible situation:
plain infinitive → present state or situation
He might know the answer.
progressive infinitive → ongoing situation
He might be sleeping.
perfect infinitive → past event or state
He might have left.
perfect progressive infinitive → ongoing past situation
He might have been sleeping.
perfect passive infinitive → past passive situation
He might have been arrested.
Note
Contrast with may: both can express possibility, but might commonly presents the possibility as more remote, tentative, or less certain.
He may be at home.
He might be at home. (more tentative)
Will¶
Minimal contrasts¶
He will leave. (future prediction; plain infinitive)
He will be leaving. (future ongoing situation; progressive infinitive)
He will have left. (future completion; perfect infinitive)
He will have been leaving. (future ongoing completion; perfect progressive infinitive)
Licensed constructions¶
He will leave. (prediction; plain infinitive)
He will arrive tomorrow. (future prediction; plain infinitive)
He will help you. (willingness; plain infinitive)
He will be at home. (future prediction; plain infinitive)
He will be sleeping. (future ongoing situation; progressive infinitive)
He will be seen tomorrow. (future passive; passive infinitive)
He will have left by then. (future completion; perfect infinitive)
He will have finished the work. (future completion; perfect infinitive)
He will have been sleeping for hours. (future ongoing completion; perfect progressive infinitive)
He will have been arrested by then. (future completion; perfect passive infinitive)
The door will not open. (prediction / tendency; plain infinitive)
He won’t come. (negative prediction / refusal; plain infinitive)
Ungrammatical¶
He will to leave.(plain form required)
He will leaves.(plain form required)
He will leaving.(plain form required)
He will left.(plain form required)
He has will leave.(no participle form)
He is will leave.(no non-finite form)
To will leave is possible.(no infinitive form)
Technical notes¶
Has a preterite form: would.
No non-finite modal forms (
to will,willingas a modal,willedas a modal).No third-person singular -s (
he willsis not modal will).Can express both prediction and volition:
He will arrive soon (prediction)
He will help you. (volition)
Will not / won’t can express refusal:
The car won’t start.
Ambiguous regarding prediction/volition:
He will help you.
Can mean either of the of:
I expect that he will help you. (prediction)
He is willing to help you. (willingness)
It depends on the context:
Don’t worry, he will help you. He is the only person who can solve it. (prediction)
Don’t worry, he will help you. He is the only person who can solve it. (prediction)
I asked him, and he said he will help you. (willingness, intention)
Perfect will have + past participle places the situation before a future reference time.
Future prediction is simply prediction with explicit future time
Interpretation clues¶
Prediction: the speaker presents a future situation as expected or likely.
He will arrive tomorrow.
Willingness: the subject is willing to perform an action.
I will help you.
Refusal (won’t): the subject refuses to perform an action.
He won’t listen.
Characteristic behaviour / tendency: describes what someone or something typically does.
Oil will float on water.
The infinitive construction locates the predicted or intended situation:
plain infinitive → future state or situation
He will know the answer.
progressive infinitive → ongoing future situation
He will be sleeping at midnight.
perfect infinitive → completed before a future reference point
He will have left by then.
perfect progressive infinitive → ongoing situation up to a future reference point
He will have been working for ten hours by then.
passive infinitive → future passive situation
The report will be published tomorrow.
perfect passive infinitive → completed future passive situation
The report will have been published by then.
Note
Willingness/refusal: the subject is normally an animate agent capable of choosing whether to act.
She will help us.
The car won’t start. (not refusal; tendency/behaviour)
Prediction: often concerns events outside the speaker’s control.
The train will arrive at six.
Note
“Will” historically expresses volition (“wanting to do something”).
Would¶
Minimal contrasts¶
I would help. (willingness; plain infinitive)
He would be helping. (remote ongoing situation; progressive infinitive)
He would have helped. (counterfactual past; perfect infinitive)
He would have been helping. (counterfactual ongoing past; perfect progressive infinitive)
Licensed constructions¶
He said he would leave. (backshifted prediction; plain infinitive)
I would help you. (willingness; plain infinitive)
I would go if I had time. (remote conditional; plain infinitive)
We would visit them every summer. (past habitual; plain infinitive)
He would be at home by now. (remote inference; plain infinitive)
He would be sleeping at this time. (remote ongoing situation; progressive infinitive)
He would be seen as a hero. (remote passive situation; passive infinitive)
He would have left by now. (counterfactual past; perfect infinitive)
She would have known the answer. (counterfactual past; perfect infinitive)
He would have been sleeping. (counterfactual ongoing past; perfect progressive infinitive)
He would have been arrested. (counterfactual past; perfect passive infinitive)
He wouldn’t help us. (unwillingness; plain infinitive)
He wouldn’t have agreed. (counterfactual past; perfect infinitive)
Ungrammatical¶
He would to leave.(plain form required)
He would leaves.(plain form required)
He would leaving. (plain form required)
He would left. (plain form required)
He has would leave. (no participle form)
He is would leave. (no non-finite form)
To would leave is possible. (no infinitive form)
Technical notes¶
Preterite form of will.
No infinitive form (
to wouldis not possible).No participial forms as a modal (
woulding,wouldedare not modal forms).No third-person singular -s (he
wouldsis not possible).Commonly expresses modal remoteness rather than past time.
Used for backshift, conditional constructions, and past habitual situations.
Perfect would have + past participle typically expresses a counterfactual past.
Interpretation clues¶
Backshifted prediction: typically follows a past reporting verb.
She said he would come.
Remote conditional: usually occurs with an explicit or implied condition.
I would go if I had time.
Willingness: the subject is willing to perform an action.
I would help you.
Past habitual: describes a repeated past situation, often with a past time frame.
Every summer, we would visit our grandparents.
Counterfactual past: refers to an unrealised past situation; often an explicit or implied condition is understood.
He would have helped if he had known.
The infinitive construction locates the remote or reported situation:
plain infinitive → backshifted prediction, remote conditional, willingness, or past habitual
She said he would come.
progressive infinitive → ongoing remote or reported situation
He would be sleeping by now.
perfect infinitive → counterfactual or unrealised past situation
He would have left.
perfect progressive infinitive → ongoing counterfactual past situation
He would have been sleeping.
passive infinitive → remote or reported passive situation
He would be seen as a hero.
perfect passive infinitive → counterfactual past passive situation
He would have been promoted.
Note
Backshift: commonly follows verbs of saying, thinking, believing, or knowing in the preterite.
She said he would come.
Counterfactual past: the context usually implies that the event did not happen.
He would have helped, but he never heard about it.
Shall¶
Minimal contrasts¶
I shall leave. (prediction; plain infinitive)
Shall I leave? (offer; plain infinitive)
Shall we leave? (suggestion; plain infinitive)
The work shall be completed. (directive; passive infinitive)
Licensed constructions¶
I shall return. (prediction; plain infinitive)
We shall overcome. (prediction; plain infinitive)
You shall have your money. (assurance; plain infinitive)
You shall obey the rules. (directive; plain infinitive)
Shall I open the window? (offer; plain infinitive)
Shall we begin? (suggestion; plain infinitive)
Shall we be waiting here? (suggestion; progressive infinitive)
The work shall be completed by Friday. (directive; passive infinitive)
They shall have finished by noon. (future completion; perfect infinitive)
The report shall have been completed before publication. (future completion; perfect passive infinitive)
I shall not forget. (negative prediction; plain infinitive)
You shall not pass. (prohibition/directive; plain infinitive)
Ungrammatical¶
He shall to leave.(plain form required)
He shall leaves.(plain form required)
He shall leaving. (plain form required)
He shall left. (plain form required)
He has shall leave. (no participle form)
He is shall leave. (no non-finite form)
To shall leave is possible. (no infinitive form)
Technical notes¶
Preterite form: should.
No infinitive form (
to shallis not possible).No participial forms as a modal (
shalling,shalledare not modal forms).No third-person singular -s (he
shallsis not possible).Restricted in contemporary English.
Primarily used with first-person subjects for prediction, and in questions expressing offers or suggestions.
Common in legal and formal language to express directives or obligations:
The tenant shall pay the rent on the first day of each month.
Interpretation clues¶
Prediction: primarily with a first-person subject in formal or traditional usage.
I shall return.
Offer: typically in a first-person singular question.
Shall I carry that for you?
Suggestion: typically in a first-person plural question.
Shall we begin?
Directive: common in legal, contractual, or highly formal language.
The tenant shall pay the rent on the first day of each month.
Assurance / promise: the speaker gives a firm assurance or guarantee.
You shall have your reward.
The infinitive construction locates the predicted or directed situation:
plain infinitive → prediction, offer, suggestion, directive, or assurance
I shall return.
passive infinitive → directive affecting the subject
The report shall be submitted by Friday.
perfect infinitive → completion before a future reference point (rare)
They shall have finished by noon.
perfect passive infinitive → completed passive situation before a future reference point (rare)
The work shall have been completed before inspection.
Note
Offers and suggestions:
Shall I …? (offer)
Shall we …? (suggestion)
Directives: usually found in legal, regulatory, or contractual texts, where shall imposes an obligation rather than predicts a future event.
Should¶
Minimal contrasts¶
You should leave. (advice; plain infinitive)
He should be leaving. (expected ongoing situation; progressive infinitive)
He should have left. (unfulfilled expectation; perfect infinitive)
He should have been leaving. (unfulfilled ongoing expectation; perfect progressive infinitive)
Licensed constructions¶
You should apologise. (advice; plain infinitive)
We should leave now. (recommendation; plain infinitive)
He should be at home by now. (expectation; plain infinitive)
She should know the answer. (expectation; plain infinitive)
He should be sleeping. (expected ongoing situation; progressive infinitive)
The report should be finished today. (recommendation; passive infinitive)
He should have arrived by now. (unfulfilled expectation; perfect infinitive)
You should have told me. (criticism/regret; perfect infinitive)
She should have known better. (criticism/regret; perfect infinitive)
He should have been sleeping. (expected ongoing past situation; perfect progressive infinitive)
The report should have been completed yesterday. (unfulfilled expectation; perfect passive infinitive)
You shouldn’t worry. (advice; plain infinitive)
He shouldn’t have said that. (criticism/regret; perfect infinitive)
Ungrammatical¶
He should to leave.(plain form required)
He should leaves.(plain form required)
He should leaving.(plain form required)
He should left.(plain form required)
He has should leave.(no participle form)
He is should leave.(no non-finite form)
To should leave is possible.(no infinitive form)
Technical notes¶
*Preterite form of shall.
No infinitive form (
to shouldis not possible).No participial forms as a modal (
shoulding,shouldedare not modal forms).No third-person singular -s (he
shouldsis not possible).Commonly expresses advice, recommendation, expectation, or weak obligation.
Perfect should have + past participle typically expresses unfulfilled expectation, criticism, or regret.
Common in mandative content clauses in formal style:
They recommended that he should resign.
Interpretation clues¶
Advice: the speaker recommends a course of action.
You should apologise.
Recommendation: similar to advice, often less personal or more general.
We should leave now.
Expectation: the speaker considers a situation likely or appropriate.
He should be at home by now.
Weak obligation: the speaker expresses moral or practical duty rather than strict necessity.
People should obey the law.
Unfulfilled expectation: the speaker expected something to happen, but it apparently did not.
He should have arrived by now.
Criticism / regret: the speaker judges that a past action was wrong or omitted.
You should have told me.
The infinitive construction locates the recommended or expected situation:
plain infinitive → advice, recommendation, expectation, or weak obligation
She should know the answer.
progressive infinitive → expected ongoing situation
He should be sleeping.
perfect infinitive → unfulfilled expectation or criticism about a past situation
He should have left.
perfect progressive infinitive → unfulfilled expectation about an ongoing past situation
He should have been sleeping.
passive infinitive → recommended or expected passive situation
The report should be completed today.
perfect passive infinitive → unfulfilled expectation about a past passive situation
The report should have been completed yesterday.
Note
Advice vs expectation:
Advice: the speaker is trying to influence what someone does.
You should apologise.
Expectation: the speaker is assessing what is probably true or appropriate.
He should be at home by now.
Note
Should have:
If the event apparently didn’t happen, the reading is usually unfulfilled expectation.
He should have arrived by now.
If the speaker is evaluating someone’s past action, the reading is usually criticism or regret.
You should have told me.