主语从句可以按其引导词的不同分为三类：

第一类，用从属连词that引导的主语从句，例如：

That we shall be late is certain.

That the driver could not control his car was obvious.

这种结构主要是对that从句的内容进行强调，属正式文体，连词that不可以省略；但是在一般情况下，往往使用先行it结构，即用it作形式主语，而把that从句放到后面，这时，在口语中，连词that有时则可以省略。所以上述两句可以改为：

It is certain that we shall be late.

It was obvious that the driver could not control his car.

如果整个句子是疑问形式，就只能用先行it结构，例如：

Is it true that he would take the risk？

Is it possible that they will come tomorrow？

常使用先行it结构，用that从句作主语的句子有下列几个句型：

1、It + be + 形容词 + that从句：

It is clear that he was telling the truth.

It's probable that we'll be a little late.

2、It + be + 名词词组 + that从句：

It's a pity that you can't go with us.

3、It + 及物动词 + 宾语 + that从句：

It worried her a bit that her hair was turning grey.

It shocked me that Peter didn't tell anybody where he was.

4、It + be + 过去分词 + that从句：

It is said that he has been there many times.

5、It + seem/happen/appear等不及物动词 + that从句：

It seems that he has lost something.

注意：

在上述第1和第2两种句型中，that从句前置与使用先行it，that从句后置在意义上没有什么差异；但使用先行it结构较为常见。

在第3种句型中，that从句前置在语法上是可能的，但实际上并不常见，通常总是使用先行it结构 .

第4种句型实质上是被动结构，由于that从句不可以位于句首作被动句的主语，所以只能使用先行it结构。

第5种句型已经形成了固定的搭配关系，that从句不能前置，只能使用先行it结构，不过，这种结构可以转换为带有不定式的简单句，例如：

It happened that I had no money with me that day.

→ I happened to have no money with me that day.