Section 14.8 Complement Clauses: Overview
Of all nominal forms, the complement clause is the most complex because it is a full clause—with its own subject and predicate—filling a single argument slot in a larger sentence. A complement clause (also called a nominal clause or noun clause) is connected to the matrix sentence by a complementizer: the word that introduces the clause and signals that what follows fills a nominal position. The most common complementizers are that, if, whether, and the wh-words what, who, whom, where, when, why, how, which.
How Complement Clauses Compare to Other Nominals.
Each nominal form adds a different degree of structural complexity. A noun phrase or pronoun fills an argument slot with a single phrase. A present participle phrase or infinitive phrase adds verbal content—an action—but lacks an internal subject of its own. A complement clause is the only nominal form that contains a full subject-predicate structure. This makes it the most flexible (it can express a complete proposition or question as a single argument), but also the heaviest, and the one most likely to create parsing difficulty in long sentences.
What Complement Clauses Express.
Complement clauses fall into two broad meaning types based on the complementizer that introduces them:
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Statements (propositions). Clauses introduced by that embed a proposition—something that can be true or false: I believe that she is honest.
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Questions (yes/no or open). Clauses introduced by whether or if embed yes/no questions; clauses introduced by wh-words embed open questions: I wonder whether she’s coming; I know what she said.
The same nominal positions appear with both meaning types. What changes is the complementizer and, with it, what the clause is doing semantically (asserting vs. asking).
Where Complement Clauses Appear.
A complement clause can fill almost any nominal slot:
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Subject: That she resigned surprised everyone.
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Direct object: I believe that she is honest.
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Subject complement: The truth is that we need more time.
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Object of preposition: We talked about what he said.
One distributional fact is worth flagging in advance: only complement clauses introduced by a wh-word can sit after a preposition. You can say about what he said but not about that he said—a constraint covered in Section 14.9.
Three Structural Patterns.
Complement clauses come in three structural patterns depending on the role the introducing word plays inside the clause. In the first, a complementizer (typically that, if, or whether) is just a marker, with a separate subject NP following it:
[CC [COMP that] [NP ...] [VP ...]]. In the second, a wh-word is the embedded subject and there is no separate subject NP: [CC [NP [PRON who]] [VP ...]]. In the third, a wh-word precedes a separate subject NP—the wh-word having been pulled from a non-subject role inside the clause: [CC [COMP what] [NP ...] [VP ...]]. Section 14.9 examines each pattern in detail with diagrams. (This three-way split parallels the structural patterns you saw for relative clauses in Section 13.10.)
