A complement clause is a subordinate clause that fills an argument position within a larger sentence — typically the object position of a verb, the subject of a verb phrase, or the complement of a noun or adjective. Complement clauses differ from relative clauses (which modify nouns) and adverbial clauses (which modify events) in that they function as noun-phrase-like arguments of the matrix (main) clause predicate.

In-Depth Explanation

Basic structure

The standard English complement clause is introduced by the complementizer that (or whether/if for indirect questions). The complementizer marks the subordinate clause’s boundary:

• I think that she left.
• She knows that it’s complicated.
• It seems that we agree.
• I wonder whether she’s coming.

The complement clause that she left functions as the direct object of think — it fills the same syntactic slot as a simple noun phrase (I think the answer).

Types of complement clauses

Matrix verb type determines which complement form is allowed: want takes infinitive (I want to go) but not that-clause (\I want that he goes — ungrammatical in English). Know takes that*-clauses and infinitives.

Japanese complement clauses

Japanese complement clauses use different marking. The complementizer to (と) introduces content/quotative complements, and ka (か) introduces indirect questions. Because Japanese is verb-final, the complement clause precedes the matrix verb:

The nominalizer no (の) also creates complement-like structures by nominalising a clause, allowing it to function as a subject or object.

Acquisition in L2

Complement clauses are among the later-acquired syntactic structures in L2 English acquisition, associated with specific verbs (think, believe, know, feel, say). Learners often omit that (“I think she left”) or use it non-targetlike. Indirect question word order in English (I don’t know where she is vs. \I don’t know where is she*) is a persistent difficulty for learners from languages without the auxiliary inversion requirement.

History

Complement clauses were analyzed within transformational grammar (Chomsky 1965, 1981) as S-bar structures dominated by complementizer (COMP) position in the X-bar schema. The Complementizer Phrase (CP) — headed by that, whether, or for — became a central node in Government and Binding theory. The semantic typology of complement-taking predicates (factives, semi-factives, implicatives) was developed by Kiparsky and Kiparsky (1970), who observed that factive verbs (know, regret) presuppose the truth of their complements, while non-factives (think, believe) do not. Cross-linguistic typology of complementation was developed by Noonan (1985).

Common Misconceptions

• “That can always be omitted in English complement clauses.” While that is optional in many subject-experiencer constructions (I think she left vs. I think that she left), omission is not always grammatical: The fact she left/\the fact she left — nominal complements often require that.
• “Complement clauses and relative clauses are the same thing.” Relative clauses modify nouns (the man who left); complement clauses fill argument positions (I know that he left). The formal similarity of embedded clauses obscures this functional difference.
• “Japanese と (to) works the same as English that.” Japanese to is a quotative/contentive complementizer and also marks quotes (reported speech). It marks complement clauses but is not used with all Japanese complement-taking predicates; some require no nominalization instead.

Social Media Sentiment

Complement clauses appear in Japanese grammar learning content in discussions of to omou (と思う, “I think that”), to iu (という, “called” / “that is called”), and indirect quotation structures — all fundamental constructions for expressing opinions, beliefs, and reported speech in Japanese. These are high-frequency teaching targets. English indirect question word order (I don’t know where it is) is a commonly discussed difficulty for Japanese learners.

Last updated: 2026-04

Practical Application

• Japanese practice: Master the と思う pattern (彼女は行ったと思う) as your first complement clause anchor. Extend to と言う (reported speech), かどうかわからない (indirect question with ka), and のが好き nominalized complements.
• English indirect questions: Practice the no-inversion rule: I don’t know where she is (not where is she). The embedded clause retains declarative word order regardless of the question interpretation.
• Factive verbs: Notice which verbs presuppose the truth of their complement (I regret that she left — implies she left) vs. non-factive verbs (I think that she left — uncertain). This matters for producing natural hedged language in academic and professional writing.

Related Terms

• Subordinate Clause
• Japanese Grammar
• Topic-Comment Structure
• Null Subject Parameter

See Also

• Sakubo – Japanese SRS App — Japanese language app; complement clause patterns like to omou and to iu are essential for reading and listening to natural Japanese and appear throughout intermediate/advanced content.

Sources

• Noonan, M. (1985). Complementation. In T. Shopen (Ed.), Language Typology and Syntactic Description, vol. 2. Cambridge University Press. — the cross-linguistic typological reference for complement clause types and complement-taking predicates across languages.
• Kiparsky, P. & Kiparsky, C. (1970). Fact. In M. Bierwisch & K.E. Heidolph (Eds.), Progress in Linguistics. Mouton. — the paper introducing the factive/non-factive distinction for complement-taking predicates; highly influential in semantics and syntax.
• Chomsky, N. (1981). Lectures on Government and Binding. Foris. — the Government and Binding theoretical framework within which complement clauses were analyzed as CP structures; foundational for linguistic theory of clause embedding.