主語從句可以按其引導詞的不同分為三類：

第一類，用從屬連詞that引導的主語從句，例如：

That we shall be late is certain.

That the driver could not control his car was obvious.

這種結構主要是對that從句的內容進行強調，屬正式文體，連詞that不可以省略；但是在一般情況下，往往使用先行it結構，即用it作形式主語，而把that從句放到后面，這時，在口語中，連詞that有時則可以省略。所以上述兩句可以改為：

It is certain that we shall be late.

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

如果整個句子是疑問形式，就只能用先行it結構，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結構，用that從句作主語的句子有下列幾個句型：

1、It + be + 形容詞 + that從句：

It is clear that he was telling the truth.

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

2、It + be + 名詞詞組 + that從句：

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

3、It + 及物動詞 + 賓語 + that從句：

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 + 過去分詞 + that從句：

It is said that he has been there many times.

5、It + seem/happen/appear等不及物動詞 + that從句：

It seems that he has lost something.

注意：

在上述第1和第2兩種句型中，that從句前置與使用先行it，that從句后置在意義上沒有什么差異；但使用先行it結構較為常見。

在第3種句型中，that從句前置在語法上是可能的，但實際上并不常見，通?？偸鞘褂孟刃衖t結構 .

第4種句型實質上是被動結構，由于that從句不可以位于句首作被動句的主語，所以只能使用先行it結構。

第5種句型已經形成了固定的搭配關系，that從句不能前置，只能使用先行it結構，不過，這種結構可以轉換為帶有不定式的簡單句，例如：

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

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