一、主語從句

1. 引導詞：主語從句就是在復合句中用作主語的從句，引導主語從句的引導詞主要有三類：一是that；二是whether；三是那些可以用作疑問詞的詞語，如what, who, which, when, where, how, why等。如：

That we shall be late is certain. 我們要晚了，這是確定無疑的。

What he says is not important. 他說的話并不重要。

How this happened is not clear to anyone. 這件事怎樣發生的，誰也不清楚。

Which side wins makes no difference to him. 哪邊贏他都無所謂。

Whether it will do us harm remains to be seen. 是否對我們有害還要看一看。

2. 形式主語：有時為了避免句子顯得“頭重腳輕”，可用it作形式主語放在句首，而把真正的主語放在句末。如：

It’s a pity that he didn’t come. 很遺憾他沒來。

It’s not yet known what she did. 她做什么還不得而知。

It was uncertain whether he could come or not. 他是否會來還不肯定。

這三句句首的it均為形式主語，相應的真主語分別是that he didn’t come / what she did / whether he could come or not。

二、表語從句

表語從句就是在復合句中用作表語的從句。一般說來，可用于引導主語從句的引導詞也可用于引導表語從句，如that, what, who, which, when, where, how, why, whether等。如：

My idea is that we should do it right away. 我的意見是馬上就干。

That is what he meant. 這就是他的意思。

The problem is how we can find him. 問題是我們如何找到他。

The morning is when I’m busiest. 早上是我最忙的時候。

That’s where you are wrong. 這就是你不對的地方。

That’s why he didn’t come. 這就是他沒有來的緣故。

另外，表語從句還可以用as if, as though, because等來引導。如：

It is because he is too foolish. 那是因為他太蠢了。

It is not as though we were poor. 又不是我們家里窮。

三、賓語從句

1. 引導詞：賓語從句就是在復合句中用作賓語的從句。一般說來，可用于引導主語從句的引導詞也可用于引導賓語從句，如that, what, who, which, when, where, how, why, whether等。如：

We believe that he is honest. 我們相信他是誠實的。

She has got what she wanted. 她要的東西得到了。

I haven’t decided whether I should go. 我還沒決定我是否會去。

I asked how he was getting on. 我問他情況怎樣。

He asked when the train would get in. 他問火車什么時候進站。

He asked me where I was going. 他問我到哪兒去。

Please tell me which you like. 告訴我你喜歡哪一個。

He asked who lived next door.他問誰住在隔壁。

She asked why he was silent. 她問他為什么一言不發。