What is Complement Clause? [6 Types]

We know that words and phrases can act as complements in a sentence:

(Comments that go with examples are in square brackets. Second, complements have been underlined.)

But clauses too can be complements. Examples:

What’s a complement clause?

A complement clause is a dependent clause that completes the meaning of a noun, adjective, verb, or preposition. It starts with conjunction known as complementizer. A complement clause is essentially a noun clause, and a complementizer is nothing but a subordinating conjunction that starts a noun clause.

Using the term ‘complement’ changes the lens through which we see the function of the clause: We now see it as a completer of the meaning of words and phrases and not as a noun filling in different slots such as subject, object, etc. (Most grammar books classify dependent clause as noun, relative, and adverb, but few classify them as complement, relative, and adverb.)

Let’s look at what all a complement clause can complement. You can tally these with different functions of a noun clause, and you’ll hardly find any difference. We’re essentially looking at the same thing through a different lens.

1. Verb-complement clause

The verb-complement clause, the most common complement clause, completes the meaning of a verb, almost always following the verb it complements. Speaking or reporting verbs such as admit, advise, agree, answer, argue, complain, inform, say, suggest, and tell commonly take complement clause. So do perception or cognition verbs such as believe, feel, guess, hear, hope, know, see, and think. But most verbs don’t. Examples:

Studies show that human productivity drops off significantly after 5-6 hours of intense mental work. [Studies show is incomplete in meaning without the underlined verb-complement clause. Also note that the clause follows the verb show.]

Scientists believe that Covid is going to stay with us for at least few years.

Some wonder if there is intelligent life like us on other planets.

We don’t know who our real friends are till adversity strikes.

Drunkenness reveals what soberness conceals.

Some grammar books also consider a clause in the subject position as complement to verb, and it makes sense. If object can be a complement to verb – the above examples – then why can’t subject be a complement to verb. After all both are required, at least by some verbs. Examples:

That Mars once had life has long been believed by scientists.

What transpired in the meeting is a secret.

How the investment fares is beyond my control.

Whether he lied under oath should be investigated impartially.

2. Preposition-complement clause

The preposition-complement clause completes the meaning of a preposition, always following the preposition it complements. Examples:

We should refrain from speculating and jumping to conclusion on why someone behaved in certain way.

The child threw tantrums at whomsoever he ran into.

Don’t copy others to fit into what others want to see.

You’ve to face the consequences regardless of who you are.

3. Adjective-complement clause

The adjective-complement clause completes the meaning of an adjective, almost always following the adjective it complements. The adjectives that are complemented by such clause follow linking verbs.

It’s not clear if the match will get a go-ahead. [It’s not clear is incomplete in meaning without the underlined adjective-complement clause. Also note that the clause follows the adjective clear, and the adjective itself follows the linking verb is.]

I’m not sure if he’ll join us for dinner.

I felt sorry that your mother met with an accident.

I’m happy that you’ve decided to come.

After a week into the tournament, it’s clear who is winning and who is not.

4. Noun-complement clause

The noun-complement clause completes the meaning of a noun, almost always following the noun it complements. This clause works with only few nouns, mainly reporting nouns such as idea, fact, comment, remark, statement, claim, argument, assumption, proof, possibility, and response. Unlike the earlier two clauses, this clause can be introduced by only that complementizer. Examples:

Nobody believed his statement that he saw a UFO. [Nobody believed his statement is incomplete in meaning without the underlined noun-complement clause. Also note that the clause follows the noun statement.]

New palaeontological discoveries have changed our belief that homo sapiens were the only human species some 30,000 years ago.

The fact that he is not present in such an important meeting speaks volumes of his interest in the project.

Earlier astronomers put forth the idea that earth was the center of universe.

His remark that Darwin’s theory of evolution is not correct expectedly created a controversy.

Noun-complement clause vs. relative clause

Noun-complement clauses can be easily confused with relative clauses as both follow a noun and both can start with that.

What’s the difference? How do you tell one from the other?

There are few differences between the two.

1. One is appositive; the other is not.

A noun-complement clause is an appositive. But a relative clause isn’t. (Whereas an appositive is a restatement of a noun, a relative clause is a modifier of a noun.) How do we use this difference to tell which is which?

Since an appositive is a restatement of noun, the noun should be the same as the appositive. For example, in the sentence U.S. President John F. Kennedy set the goal of landing a human on moon, where John F. Kennedy is appositive, we can say:

U.S. President = John F. Kennedy (Or can U.S. President be John F. Kennedy? Yes.)

Let’s try the same with the two types of clauses.

Only the first makes sense: Earth was the center of universe can be an idea. Hence, only the first of the three examples contains a noun-complement clause.

You can try this with other examples of noun-complement clause we covered earlier.

2. That is not a clause constituent in one; it is in the other

That is not a clause constituent in noun-clause complements and hence is not required for the clause to make a meaningful sentence. In the first example, earth was the center of universe doesn’t require that to form a meaningful sentence.

However, that is a clause constituent in relative clauses and hence is required for the clause to make a meaningful sentence. In the second example, that (= telescopes) is the subject of the clause and is required to convert the clause into a meaningful sentence: Telescopes couldn’t tell much about the universe. In the third, that (= telescopes) is the object of the clause and is required to convert the clause into a meaningful sentence: A layperson couldn’t use telescopes at all.

3. That can never be dropped in one of them; it can sometimes be in the other.

That can never be dropped in a noun-complement clause but can sometimes be in a relative clause. If that is the subject of a relative clause, it can’t be dropped. Otherwise, yes. Examples:

So, this test can differentiate between the two if the relative clause has a subject other than that.

4. That is always followed by a noun in one and rarely in the other.

In noun-complement clause, that will always be followed by a noun. But in case of relative clause, that will be rarely followed by noun.

5. One can come with only limited set of nouns; the other with any.

A noun-complement clause can come with very limited set of nouns (we covered few earlier). A relative clause though can come with any noun.

5. Subject-complement clause

A subject-complement clause completes the meaning of subject of a sentence. In this role, it follows the linking verb. Examples:

The answer isn’t what you think. [The clause adds more information to the subject the answer.]

Our problem is whom we trust.

The problem is that the test is not full-proof.

6. Object-complement clause

An object-complement clause follows the direct object and complements it. Example:

We nicknamed John what some may think to be offensive. [The clause gives further meaning to the direct object John.]