A that-clause is an example of a noun clause. It can be the subject or the object of the verb.
I believe that he is innocent.
Here the that-clause ‘that he is innocent’ is the object of the verb believe.
She said that she can speak three languages. (Object – that she can speak three languages)
I suspect that she eloped with her boyfriend.
The that-clause can also act as the subject of the verb.
Study the examples given below.
That she should forget me so quickly was rather a shock.
Here the that-clause ‘that she should forget me so quickly’ acts as the subject of the verb was.
That she delivered a marvelous performance pleased her parents. (Subject – that she delivered a marvelous performance)
Instead of ‘that’ we can also use the expressions ‘the fact that’, ‘the belief that’, ‘the idea that’ and ‘the evidence that’.
The fact that she didn’t recognize me was rather a shock.
The fact that she didn’t understand English made it difficult for her to get a job.
The idea that the teacher should know everything is unacceptable.
If the subject is too long, it may be difficult for the reader to understand the sentence. To solve this problem, we usually use the introductory ‘it’.
In this case, ‘it’ will occupy the position of the subject, but it is not the real subject. It merely signals that a noun-clause is to follow.
Examples are given below.
It surprised me that he was still in bed. (More natural than ‘That he was still in bed surprised me.’)
She made it clear that she wouldn’t accept the proposal.
(NOT She made that she wouldn’t accept the proposal clear.) (NOT She made clear that she wouldn’t accept the proposal.)
Note that the introductory it can be used only with ‘that’. It cannot be used with the expressions ‘the idea that / the belief that / the fact that’ etc.