一、主语从句

1. 引导词：主语从句就是在复合句中用作主语的从句，引导主语从句的引导词主要有三类：一是that；二是whether；三是那些可以用作疑问词的词语，如what, who, which, when, where, how, why等。如：

That we shall be late is certain. 我们要晚了，这是确定无疑的。

What he says is not important. 他说的话并不重要。

How this happened is not clear to anyone. 这件事怎样发生的，谁也不清楚。

Which side wins makes no difference to him. 哪边赢他都无所谓。

Whether it will do us harm remains to be seen. 是否对我们有害还要看一看。

2. 形式主语：有时为了避免句子显得“头重脚轻”，可用it作形式主语放在句首，而把真正的主语放在句末。如：

It’s a pity that he didn’t come. 很遗憾他没来。

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

It was uncertain whether he could come or not. 他是否会来还不肯定。

这三句句首的it均为形式主语，相应的真主语分别是that he didn’t come / what she did / whether he could come or not。

二、表语从句

表语从句就是在复合句中用作表语的从句。一般说来，可用于引导主语从句的引导词也可用于引导表语从句，如that, what, who, which, when, where, how, why, whether等。如：

My idea is that we should do it right away. 我的意见是马上就干。

That is what he meant. 这就是他的意思。

The problem is how we can find him. 问题是我们如何找到他。

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

That’s where you are wrong. 这就是你不对的地方。

That’s why he didn’t come. 这就是他没有来的缘故。

另外，表语从句还可以用as if, as though, because等来引导。如：

It is because he is too foolish. 那是因为他太蠢了。

It is not as though we were poor. 又不是我们家里穷。

三、宾语从句

1. 引导词：宾语从句就是在复合句中用作宾语的从句。一般说来，可用于引导主语从句的引导词也可用于引导宾语从句，如that, what, who, which, when, where, how, why, whether等。如：

We believe that he is honest. 我们相信他是诚实的。

She has got what she wanted. 她要的东西得到了。

I haven’t decided whether I should go. 我还没决定我是否会去。

I asked how he was getting on. 我问他情况怎样。

He asked when the train would get in. 他问火车什么时候进站。

He asked me where I was going. 他问我到哪儿去。

Please tell me which you like. 告诉我你喜欢哪一个。

He asked who lived next door.他问谁住在隔壁。

She asked why he was silent. 她问他为什么一言不发。