That-Clauses and Whether-Clauses as Complements.

A complement clause completes the meaning of a verb, adjective or noun. Use that-clauses to present statements and whether-clauses to present yes/no questions or alternatives. The choice shows whether you are presenting an idea or considering if it is true.

1.Statements and open questions

Compare these two sentences:

A that-clause gives the content of a statement, belief, feeling or claim. It does not necessarily describe a fact:

• Maya believes that the plan will work. → This is her belief, not a guarantee.
• They denied that they had broken the window. → This is what they denied.

A whether-clause presents a yes/no question or a choice between alternatives. It does not always mean that the speaker is uncertain:

• I don't know whether Sam is coming. → I do not have the answer.
• I know whether Sam is coming, but I can't tell you. → I have the answer, but I do not give it.

2.Form and word order

Both types normally use subject + verb order, even when a whether-clause represents a question.

Clause type	Structure	Example
Statement content	that + subject + verb	She explained that the train was delayed.
Yes/no question content	whether + subject + verb	She asked whether the train was delayed.
Negative statement content	that + subject + negative verb	I think that he doesn't agree.
Negative question content	whether + subject + negative verb	I wonder whether he hasn't seen the message.

Compare a direct question with a complement clause:

The second complement appears inside a question, but its own word order remains the door is, not is the door. The question mark belongs to the whole question.

3.Complements after different words

Complement clauses can follow verbs, adjectives and nouns. The word before the clause affects which type is possible.

Position	That-clause	Whether-clause
After a verb	We believe that the figures are correct.	We discussed whether the figures were correct.
After an adjective	I'm pleased that you can come.	I'm unsure whether you can come.
After a noun	The claim that the product is safe needs evidence.	The question of whether the product is safe remains unanswered.
After the linking verb be	The problem is that we have no money.	The question is whether we can afford it.

With question and issue, of whether is a common pattern: the question of whether it is safe, the issue of whether we should proceed. The question whether is also possible, especially in formal writing. With doubt, use doubt about whether or doubt as to whether: There is some doubt about whether it will work.

Useful patterns include:

• think, believe, say, explain + that-clause;
• ask, wonder, discuss + whether-clause;
• certain, pleased, aware + that-clause;
• unsure, uncertain + whether-clause.

Other common B2 patterns are:

Pattern	Example
suggest + that	The evidence suggests that the figures are wrong.
realise + that	I realised that I had left my keys at home.
admit + that	She admitted that she had made a mistake.
decide + whether	We must decide whether we can afford the trip.
find out + whether	Please find out whether the tickets are still available.
doubt + whether	I doubt whether we have enough time.
doubtful + whether	It's doubtful whether the repairs will be finished today.
not sure + whether	I'm not sure whether this is the right address.

Some words allow both, with a difference in meaning:

Do not assume that every verb accepts both patterns. For an open question, say I wonder whether she agrees, not I wonder that she agrees.

3.1Noun complements and relative clauses

Compare these two noun phrases:

• the claim that the product is safe → The clause gives the content of the claim. The product is safe is a complete statement.
• the claim that the company made → The relative clause identifies the claim. The company made the claim, so that represents the object of made.

Only the relative clause allows which here: the claim which the company made. Because that is the object in this relative clause, you can also omit it: the claim the company made.

In a noun complement, that introduces the content and cannot be replaced by which. Keep it in the claim that the product is safe for clarity. Some noun complements allow omission, especially informally, so omission alone is not a reliable test for a relative clause.

4.When you can leave out that

After many common verbs, that is optional, especially in conversation:

• I think that you're right. → I think you're right.
• She said that she was tired. → She said she was tired.

It is also often omitted after common adjectives:

• I'm glad that you're here. → I'm glad you're here.

Keep that when it makes a long sentence easier to follow. In formal writing, it is often helpful after nouns: the claim that the product is safe.

5.Whether, if and alternatives

After many verbs, if can replace whether when introducing a yes/no question. Whether is often preferred in formal writing.

• I don't know whether the shop is open.
• I don't know if the shop is open.

However, they are not interchangeable in every structure.

Context	Pattern to use	Example
After a preposition	whether	It depends on whether we have time.
Immediately before or not	whether or not	I don't know whether or not she agrees.
After a verb, including with or not at the end	whether or if when the verb allows both; use whether after discuss and consider in standard edited English	I don't know if she agrees or not. We discussed whether we should stay or not. We considered whether we should leave.
Before an infinitive	whether to	We haven't decided whether to stay.

For two explicit alternatives, whether … or … makes the choice clear:

6.Clauses as subjects and extraposition

That-clauses and whether-clauses can also act as the subject of a sentence:

• That he lied is obvious. → The fact that he lied is obvious.
• Whether he comes doesn't matter. → His coming or not coming makes no difference.

Keep that when a that-clause is the subject before the main verb. For a yes/no clause in this position, use whether, not if:

✗ If he comes doesn't matter.

✓ Whether he comes doesn't matter.

It is often more natural to begin with it and put the clause later, especially when the clause is long. This pattern is called extraposition. Here, it fills the subject position without referring to a particular thing.

Clause as subject	Extraposition with it
That the plan failed is clear.	It is clear that the plan failed.
Whether they will agree is unclear.	It's unclear whether they will agree.
That he lied is obvious.	It is obvious that he lied.
Whether he comes doesn't matter.	It doesn't matter whether he comes.

The restriction on if applies to a subject clause before the main verb. After introductory it, if is possible in some patterns: It's unclear if they will agree. Whether works in both positions.

7.Quick decision checklist

1. Are you presenting a statement, belief or claim? Choose that: I believe that it's possible.
2. Are you presenting a yes/no question or alternatives? Choose whether: I wonder whether it's possible.
3. Check the preceding word: believe that, but wonder whether.
4. Use subject + verb order inside the clause.
5. Omit that only where the pattern allows it; do not omit whether.
6. After a preposition or before to + verb, choose whether, not if.