Mind the gap

English allows concise constructions, that structurally leave gaps in the sentence. This helps to focus and emphasize the remaining parts.

Hollow clause

A clause with an internal gap whose interpretation is supplied by something outside the clause.

In the following sentence, the matrix subject (“this tool”) is understood as an argument inside the infinitival clause.

This tool is easy to use

Although “use” is a transitive verb, in this sentence it has no object - the gap: to use __. The object is syntactically “required” by the verb, but missing.

English allows it:

This tool is easy to use it (we don’t need the “it”)

The matrix’s subject, “this tool”, “fills” the object gap of the subordinate clause (…to use [the tool]).

Why it is “Hollow” and not just “Absent?”

CGEL uses the term hollow because the clause has a “hole” in it that must be filled by something outside of that clause.

The matrix subject (“this tool”) is forced to do two jobs at once:

• It is the Subject of the whole sentence (This tool is easy…).

• It “fills” the Object gap of the subordinate clause (…to use [the tool]).

Summary: The object is “hollow” because the grammar of the verb use demands an object, but the rules of the easy-type construction forbid you from pronouncing it. Instead, the sentence leaves a gap that points back to the matrix subject.

Types of hollow clauses

Type	Clause form	Function	Example (gap shown)	Gap is a slot for	What fills / corresponds to the gap
Relative clause	Finite	Modifier of NP	the book [I read __]	Object of read	the book
Interrogative (matrix)	Finite	Main clause	What did you buy __?	Object of buy	what
Interrogative (embedded)	Finite	Complement	I wonder [what she bought __]	Object of bought	what
Comparative clause	Finite	Comparative complement	She is taller [than I expected __]	Predicative complement (understood)	taller (than expected standard)
Adj + infinitive	Non-finite	Complement of Adj	This tool is easy [to use __]	Object of use	this tool
N + infinitive	Non-finite	Modifier of N	a book [to read __]	Object of read	a book
Preposition stranding	Finite	Relative / interrogative	the person [I spoke to __]	Complement of to	the person
Adjunct infinitival	Non-finite	Adjunct	He arrived [__ to find the door locked]	Subject of the infinitival clause	he

Relative clause

the book [I read __]

→ gap = object of read
→ filled by: the book

Interrogative (matrix clause)

What did you buy __?

→ gap = object of buy
→ filled by: what

Interrogative (embedded clause)

I wonder [what she bought __]

→ gap = object of bought
→ filled by: what

Comparative clause

She is taller [than I expected __]

→ gap = predicative complement (understood “tall”)
→ filled by: comparison with matrix property (taller than expected)

Adjective + infinitival complement

This tool is easy [to use __]

→ gap = object of use
→ filled by: this tool

Noun + infinitival complement

a book [to read __] → gap = object of read → filled by: a book

Preposition stranding (relative/interrogative)

the person [I spoke to __]

→ gap = complement of to
→ filled by: the person

Adjunct infinitival (borderline case)

He arrived [__ to find the door locked]

→ gap = subject of the infinitival clause → filled by: he

Unexpressed subject

This looks like a gap, but not a real gap.

The object gap is a hollow element. It is mandatory; the sentence fails if you don’t have a matrix subject to fill it.

The unexpressed subject is optional. It doesn’t need to be “filled” by a noun in the sentence. It can be retrieved from context or simply understood as “general” (people in general).

When using infinitive VP, we can omit the subject:

This tool is easy to use

We could say

This tool is easy to use by anybody

And the canonical form:

Anybody finds this tool easy to use (“easy” is still a predicative complement. We don’t need “easily”, which is a manner adjunct)

We could say by “Anybody” or “People”, but we don’t have to. This gives more emphasis to what we are focus on - the tool.

Reduced relative clause and hollow clauses

“Reduced relative clause” is a form distinction (finite vs non-finite), while “hollow clause” is a structural property (presence of a gap). They cross-classify: Some reduced relatives are hollow, some are not

the book [written __ by Kim]

→ gap = object of written
→ filled by: the book
→ hollow clause

The man [standing by the door]

→ no missing argument
→ no gap
→ not hollow ❌

Reduced relatives belong under relative clauses (#1) in the table, but they count as hollow clauses only when they contain a gap.

Ellipsis

Something present in structure but unspoken, recoverable by copying

Jill works in Paris, and her husband __ in Milan (missing “works”)

This is coordination ellipsis, specifically:

• Predicator omission in the second coordinate

• Recoverable from the first clause (works)

We can just copy the recoverable element:

Jill works in Paris, and her husband works in Milan

But not in a hollow clause:

This tool is easy to use this tool

Ellipsis omits words; a gap omits a syntactic position.

Restriction to passive voice gap

Usage of infinitival with specific verbs can make a gap that is not filled at all.

He is said to be rich.

Structure:

• He = subject of matrix clause

• to be rich = infinitival clause

• he is also understood as subject of be (“gap”)

People say him to be rich

Because say + infinitival (with this sense) is restricted to the passive construction. “Say” does not license an object + infinitival complement in this meaning.

Other verbs do:

They believe him to be rich (grammatical)

They consider him to be smart (grammatical)

And the passive with the gap:

He is believed to be rich.

He is considered to be rich.

Note about ECM

ECM = Exceptional Case Marking.\

The term comes from generative grammar. It describe constructions like “I believe him to be honest”. The subject of an embedded infinitival verb seems to appear in a superordinate clause and, if it is a pronoun, is unexpectedly marked with object case morphology (him not he, her not she, etc.). The unexpected object case morphology is deemed “exceptional”.
A verb assigns accusative case to the subject of an embedded non-finite clause. Huddleston and Pullum analyze these structures as raised object constructions or complex-transitive structures.

CGEL classifies these verbs (like believe, expect, want) as Raising-to-Object verbs. Here is the structural breakdown:

• The Matrix Clause: I believe him...

• The Subordinate Clause: ...to be honest.

In this view, the subordinate clause is hollowed out of its subject. The “subject” has been raised into the matrix clause to function as its object.