Complement clause
Definition
A complement clause completes the meaning of a verb, adjective or noun. It contains its own subject and verb and can be introduced by that, whether, if or a wh- word.
Examples
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I think that she is ready.
The that-clause completes the meaning of think.
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We were unsure whether the shop was open.
The whether clause completes the adjective unsure.
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The question of why the system failed remains unresolved.
The why clause supplies the content of the noun question.
Complement clause vs. adverb clause
I stayed home because I felt ill.
The clause adds the reason for the main action.
I knew that I felt ill.
The clause supplies the content of what was known.
An adverb clause adds circumstances such as time, reason or condition. A complement clause fills a meaning slot required or licensed by a verb, adjective or noun.
That can sometimes be omitted
After many common verbs, especially in informal language, that may be left out. Embedded wh- and whether/if clauses still use statement word order.
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I think she is ready.
The omitted that is understood after think.
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Do you know where the keys are?
The embedded clause uses statement order, not direct-question order.
Common mistakes
I wonder where are the keys.
I wonder where the keys are.
An embedded question uses statement word order.
She explained me that the office was closed.
She explained to me that the office was closed.
Explain does not take a person directly as its object in this pattern; use explain to someone that....
Quick check
Question 1 of 3
Which sentence contains a complement clause after a verb?
That the figures are accurate supplies the content of believe.Which embedded clause has the correct word order?
An embedded why clause uses statement order: why the door is locked.Which sentence contains a complement clause after an adjective?
That the plan succeeded completes the meaning of pleased.Nice work — you've completed the quick check.
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