it为后面不定式的先行词

it为后面不定式的先行词
一、作形式主语
主要用于It is+ adj. n. + doingto do从句句型。

Is it necessary for me to do so? 我有必要这样做吗？
It is silly of him to do so. 他这样做真是可笑。

It’s no good waiting here. 在这里等没有用。

Is it certain that they will win? 他们一定会赢吗?
It’s my duty to help you improve English. 帮助你们提高英语水平是我的职责。

注意：与it作形式主语的句型易混的句型。

It is known to all that the earth goes around the sun. 众所周知，地球围绕太阳旋转。

As is known to all, the earth goes around the earth.
It is difficult to persuade him to stay. 劝服他留下来很难。

What is difficult is to persuade him to stay. 困难的是要劝服他留下来。

二、作形式宾语
1. vt. (find, think, make, etc. ) + it +adj. n. +doing to dothat…
He made it a rule never to eat between meals. 他不吃零食已成了习惯。

I think it impossible for us finish our work on time . 我认为我们不可能准时完在这项工作。

We found it no good arguing with him. 同他争论是没用的。

I’ll make it clear to you that failure is the mother of success. 我要告诉你们失败的成功之母。

I’d think it well worth while to go. 我觉得很值得一去。

2. vt. (enjoy, like, love, hate, prefer, appreciate, take, etc. ) +it+ if when
I don’t like it if you arrive late. 我不喜欢你迟到。

She hates it when he uses her pen. 她讨厌他用她的钢笔。

I take it you have been out. 我猜你刚出去回来。

You can put it that it was arranged before. 你可以说这是以前安排的。

3. see to it thatdepend on it that(因介词后通常不接that从句，故用it作形式主语)
See to it that you are not late again. 注意别再迟到了。

You may depend upon it that we shall never lose heart. 你放心我们绝不会灰心。

4. take it for granted that等I took it for granted that you would stay with us. 我肯定你会住在我们这里的。