一、基本用法

当不定式、动名词、从句等用作主语时，避免头重脚轻，通常在句首使用形式主语it，而把真正的主语放在句子末尾：

Is it necessary to tell his father everything? 有必要把这一切都告诉他父亲吗?

It's no good sitting up too late. 熬夜没有好处。

It is quite clear that he has read the book. 很显然，他读过这本书。

It doesn't matter what he says. 他说什么没关系。

二、几种特殊的形式主语

(1) 用作动词look, seem, appear, happen, occur, follow等的主语(后接that从句或as if从句)：

It seems as if it is going to rain. 看来要下雨了。

It seems that he knows everything. 他好像什么都知道。

It (so) happened that I had no money with me. 碰巧我没带钱。

From this witness it follows that he must be guilty. 根据该证人的证词判断，他一定有罪。

(2) 当系表结构后接有if 或when引出的状语从句时，通常在系表结构前使用形式主语表示说话人对某情况的看法：
It was a great surprise to me when he did a thing like that. 他这样做使我很吃惊。

It would be a tragedy if anything went wrong. 如果出了什么事就是一个悲剧。

(3) 用于其他固定句型，如if it were not for…(若不是因为)，it's time that…(该……了)等。

三、误用形式主语的几种情形

(1) 在该用it作形式主语的地方而误用其他词：

Is ______ necessary to tell his father everything?

A. it B. that

C. what D. he

此题的正确答案应是A(it用作形式主语)，而不能选D。

(2) 在不该用形式主语的地方而误用形式主语：

______ must be something wrong with the machine.

A. There B. It

C. He D. That

此题应选A，为there be句型与情态动词must连用的情形。

比较以下两组句子：

______ is known to us all, the earth turns around the sun.

______ is known to us all that the earth turns around the sun.

A. It B. Which

C. As D. That

前一句填C，as用以引导一个非限制性定语从句；后一句填A，it为形式主语，真正的主语是其后的that从句。

______ is difficult to persuade her to stay.

______ is difficult is to persuade her to stay.

A. It B. What

C. He D. That

前一句填A，it为形式主语；后一句填B，what用以引导主语从句。注意后一句比前一句在不定式前多了个is。