It充當形式主語或者形式賓語時，可替代以下東東：
一、形式主語
① 主語從句、不定式多用形式主語it
- To learn English well is not easy. = It is not easy to learn English well.
- Whether you agree or not doesn't matter. = It doesn't matter whether you agree or not.
- It's likely that you will win.
- It's believed that he can finish the task alone.
② 疑問、感嘆用形式主語it
- How easy it is to finish the task!
- Is it possible for him to do the work alone?
③ 復合結構用形式主語it
- It is foolish of him to cheat in the exams.
- It is difficult for us to distinguish the twins.
- It is unwise to press the child too hard.
- It is necessary to simplify the problem.
- It is wrong of you to judge a person by his appearance.
④ 動名詞用形式主語的不多
- Swimming is my favorite sports in summer.
- Collecting stamps is his hobby.
復合結構 sb's doing
- Her learning English well enabled her to get a good job.
- The boy's cheating in the exam cost him too much.
*當表語是no use, no good時，形式主語it 更好
- It's no use arguing with him.
- It's no good regretting.
- Is it any use shouting like that?
⑤ what 從句不用形式主語it
- What you said hurt his feelings.
- What you need is some help.
- I could not understand what he said.
註意： 形式主語it和強調句的it不要混淆！！！
二、形式賓語
It作形式賓語可替代非謂語動詞與從句
① Vt + it + Oc + O
- I feel it hard to finish the work today.
- He feels it his duty to help others.
- I think it necessary to talk with him.
- I think it foolish of you to cheat in exams.
- I made it clear that I would never give up.
- I took it for granted that I had failed.
* adj.隻能以不定式作主語，不能以人作主語的，用for; adj. 能以不定式和人作主語的，用of
- I think it kind of you to help me.
- I think it convenientfor you to take exercises in the gym nearby.
② 動詞賓補是no use, no good用it
- I find swimming relaxing. （不用it）
- I find it no use learning like that.
③ Vt + O用it 做形式賓語的情況
- I would appreciate it if you give a hand.
- I hate it when you talk to me with your mouth full.
