That the earth is round is true. = It is true that the earth is round.
That you can not come to my wedding is a pity.
= It is a pity that you can not come to my wedding.
It's a bad habit that John goes to bed without brushing his teeth.
如句中有how, when, where, why,或whether等疑问词所引导的子句，也可以用it代替，再把子句移到后面。
How he escaped from jail is unknown.
= It is unknown how he escaped from jail. 没人知道他是怎么越狱的。
Why she was killed is still a puzzle.
= It is still a puzzle why she was killed. 她的被害仍然是个谜。