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A complement clause is a subordinate clause (a clause that cannot stand alone as a sentence) that complements or completes the meaning of a verb, adjective, or noun in the main clause. Complement clauses usually function as the subject, object, or subject complement of the sentence and provide necessary information that completes the thought introduced by the principal verb or a linking verb. They complete or complement the meaning of certain verbs, adjectives, or nouns in the main clause. Complement clauses provide additional information, often acting as the direct object, subject, or complement of the main clause.

Types of Complement Clauses


These clauses often follow verbs such as “say”, “think”, “know”, “believe”, and “hope”, or adjectives like “certain” and “glad.” They begin with the word “that”, which can sometimes be omitted.

E.g., She believes (that) she can win the race.


These can act as complements and contain a wh-word such as “what”, “who”, “how”, “why”, “when” or “where.”

E.g., I don’t know what he wants.


These clauses express conditions or indirect questions.

E.g., She is wondering whether/if John will come to the party.

Infinitive Clauses

These clauses begin with “to” and contain an infinitive verb. They can serve as the subject, object, or complement of a sentence.

E.g., To read more is her goal. (subject)
He wants to read more. (object)
Her goal is to read more. (subject complement)


These clauses typically contain a verb in the “ing” form and can act as a subject, object, or modifier (adjective or adverb) in a sentence.

E.g., Swimming in the ocean can be dangerous. (subject)
I enjoy swimming in the sea. (object)
The man swimming in the sea is a lifeguard. (adjective modifier)

The tourists left the beach, feeling refreshed by the cool sea breeze. (adverbial modifier)

Subject Complement

They often follow a linking verb and describe or provide more information about the subject.

E.g., Her dream is to compete in the Olympics.

Roles in the Sentence

Noun Complement Clauses

  • Function as subjects, direct objects, indirect objects, or objects of prepositions.
  • Usually introduced by subordinating conjunctions like “that,” “whether,” “if,” or “why.”

E.g., What you said is important. (subject)
I don’t know if I can come. (direct object)
Tell me what you need. (indirect object)
I’m worried about whether she’ll be okay. (object of preposition)

Adjective Complement Clauses

  • Modify nouns or pronouns.
  • Often introduced by “that” or relative pronouns like “who,” “which,” “whose,” and “whom”.

E.g., I’m looking for a book that explains complement clauses.
The person who helped me was very kind.

Adverb Complement Clauses

  • Modify verbs, adjectives, or other adverbs.
  • Often introduced by subordinating conjunctions like “how,” “where,” “when,” “why,” and “as.”

E.g., I’ll do it as you suggested.
She explained why she was late.
They left before I arrived.

Complement clauses often begin with subordinating conjunctions or relative pronouns such as “that,” “if,” “whether,” “what,” “how,” etc. The choice of a subordinating element depends on the specific requirements of the verb, adjective, or noun in the main clause. Complement clauses are important for constructing complex sentences, allowing speakers and writers to express complete ideas with appropriate detail.