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.  

That Leonard is very handsome is undebatable

➩ It is undebatable that Leonard is very handsome.

What he says is not important.  

How he managed is unknown.

Where he lives is unknown.

What I need is money.

Who he loves is unknown.

Whether it will do us harm remains to be seen.  

Whether he will come or not is unknown. 

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。

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.

That is what he meant.  / that is  what  he wants

The problem is how we can find him.  

The problem is where we live together.

The problem is how we get back to our dormitory.

That is why he left.

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.  / it is because things are complicated

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

He sleeps as if he hasn’t slept for a month.

We cherish every moment as if it ends tomorrow.

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.    I wonder where he lives

She has got what she wanted.  

She does not know where to go

I haven’t decided whether = if I should go.  

He asked when the train would get in. 

2. if与whether：if通常不用于引导主语从句或表语从句，但可以用于引导宾语从句，与whether用法相同，常可互换。如：

I wonder.

Is the house large enough.

I wonder whether /if the house is large enough.

She asked. 

Will John come tomorrow.

She asked whether John would come the next day.

但是，当于用于介词后引导宾语从句时，只能用whether，不能用if。如：

I worry about whether I hurt her feelings.  

There has been a long debate as to (/about) whether higher education should be free or not. 关于大学教育是否应该免费，人们一直争论不休。

Should schools teach students good behaviors? People debate.

People debate as to / about whether schools should teach students good behaviors.

3. that的省略：that引导主语从句或表语从句时通常不省略，但引导宾语从句时通常可以省略。如：

She said (that) she didn’t want to know.

I don’t think (that) she’s interested in it. 

4. 否定转移：当主句主语为第一人称，且主句谓语动词为think, believe, suppose, expect, imagine等时，若其后接一个表示否定意义的宾语从句，此时通常应将否定转移到主句上。如：

I don’t think that he will leave.

I don’t believe that it is easy.  

四、同位语从句

在复合句中用作同位语的从句叫同位语从句。所谓同位语，就是指两个语法单位指同一个人或事物，并且句法功能也一样，此时我们就把后一项称为前一项的同位语。所谓同位语从句，就是指用于名词后, 对该名词的内容进行说明解释的从句。如：

Tom, my best friend, is very tall.

Mary suddenly got the feeling (that someone was watching her.) 同位语从句

Mary suddenly got the feeling ( that was about Tom. ) 定语从句

英语中只有少数名词可后接同位语从句，比较常见的有belief, doubt, fact, feeling, hope, idea, message, news, opinion等

另外，同位语从句的引导词也比较有限，主要是由that引导，有时可也可用when, how, where, whether, who等引导，但是不能用if也引导，也不能用what来引导。如：

You have no idea how worried I was!  

He had no idea why she left.  

There is some doubt whether he will come.  

I was surprised by the news that Tom lost. 同位语从句

I was surprised by the news (which is about Tom. )