Distance

Restriction of modal auxiliary to bare infinitival

She can do it

She can to do it

The restriction of modal auxiliaries to bare infinitivals—rejecting the subordinator to—is not an arbitrary historical accident.

It is the direct consequence of how semantic fusion, functional economy, and the original lexical nature of to interacted during the grammaticalization process.

To understand why the pathway evolved this way, we must examine the historical semantics of the infinitival marker itself and why it proved incompatible with modal operators.

The original semantic role of “to”

 Direction and goal, purpose

In Old English, “to” was not a meaningless grammatical glue; it was a full spatial preposition meaning ”toward” or “at”.

When “to” began appearing with infinitives (originally dative case noun-forms ending in -enne), it retained its directional core.

It framed the infinitive as a goal, purpose, or unrealized future action toward which a subject was moving.

This explains why verbs expressing desire, attempt, or future intent naturally attracted to-infinitives:

He wants to go. (the action of going is a conceptual goal toward which his desire is directed).

She tries to win. (the winning is a target located in the future).

Because “to” establishes a directional gap between the matrix verb and the subordinate action, it requires a semantic “space” where an agent strives, intends, or moves toward a goal.

Modals and the absence of conceptual distance

Not directness: ability, necessity, possibility

Core modals (can, must, may, shall) operate entirely differently. They do not express a movement toward an action; they express an intrinsic, instantaneous state of ability, necessity, or possibility that inheres directly in the event itself.*

He must go (no temporal or spatial gap between the obligation and the departure)

The necessity is fused inextricably with the action.

Because modals act as functional operators that modify the core truth-value of the proposition instantly, inserting a directional marker like to would introduce a semantic contradiction.
It would treat the action as a distant, unrealized goal rather than an immediate modality.

Historically, because preterite-present verbs (cunnan, magan) originally functioned as full lexical verbs denoting inherent mental capacity or possession, they governed their complement verbs directly, without conceptualizing them as directional goals.

They never required the prepositional bridge

The economics of grammaticalization

 Semantic bleaching drives structural erosion

“to know how” >> “to be capable”

As modals bleached of their heavy lexical meanings () and reanalyzed into functional heads, they formed an increasingly tight structural bond with their lexical verbs.

Verbs that maintained a semantic gap (requiring volition, intention, or temporal sequence) retained the intervening subordinator “to”.

Modals, which fused completely with their predicates to form a single, inseparable verbal complex, discarded any need for an intervening link.

The structural barrier “to” was stripped away because the dependency between the modal and the plain-form verb became absolute.

A unified predicate

By rejecting “to”, modal constructions bypassed the subordination hierarchy altogether.

Instead of embedding a separate, goal-oriented clause, a modal and a bare infinitive weld together into a complex single predicate.

The modal provides the tense and the epistemic/deontic lens, while the bare infinitive provides the core event, operating as a single, indivisible cognitive unit for the reader.

Modal and plain form verb dependency

No longer operate as a matrix verb

When the dependency between a modal and a plain-form verb becomes absolute, it means the two elements have fused so completely that they no longer operate as a matrix verb governing a separate subordinate clause.

Instead, they function as a single, indivisible verbal complex.

The Dissolution of the Clause Boundary

Single two-parts predicate

In looser structures (such as catenative verbs like “want” or “try”), there is a structural seam between the matrix verb and the complement:

He wants [ to go ] (the bracketed segment retains the ghost of a separate clause, signaled by the subordinator “to”)

With modals, that seam completely vaporized.

A construction like “must go” does not contain an embedded clause; it contains a single clause with a complex, two-part predicate.

The modal and the lexical verb share the same syntactic space without any structural buffer.

Immunity to Separation

No element between the modal and it's verb

There is a ban on interlopers. An absolute dependency means the connection is so tight that no syntactic material can wedge its way between the modal and its verb.

must to go.

You cannot introduce a nominal complement directly between them without disrupting the unit.

She must analyze [the evidence].

She must the evidence analyze. (placing the nominal complement directly between the modal and the verb destroys the structural unit).

The modal and the plain-form verb are locked in a strict linear order where nothing can interrupt the government of the head over its complement.

Mutual semantic and grammatical co-dependence

Neither element can stand alone

Neither element can stand alone or fulfill its function without the other:

The modal is functionally incomplete: It carries tense and modality, but it has no independent semantic content of its own. It must govern an action.

The verb is morphologically stranded: Stripped of tense, agreement, and the infinitival marker to, the plain-form verb cannot exist as a predicate on its own.

It relies entirely on the modal above it to anchor it into reality.

Note

Historically, as these verbs grammaticalized, they stopped looking outward toward an object or a distinct goal-oriented action (to) and turned inward, locking themselves directly onto the core meaning of the lexical verb.

The result is a compressed, monolithic tool where the modal’s lens and the verb’s action strike the reader simultaneously.