1． 主语从句：主语从句在整个句子中作主语。

(1)that不同于其它的连词，它不充当句子的成分，只是单纯的连接词，通常不可以省略。从句置于句首时，that绝对不可以省略。

That the driver could not control his car was obvious.

(2) if不能用在主语从句中，而是用whether

Whether he left (or not) is unknown.

(3)当主语从句作主语时，谓语动词一般用第三人称单数，下面这个句型例外。

What we need is more time and money.

What we need are many more books.

(4)形式主语it代替后面的主语从句

It’s surprising that the little dog can work out math problems.

       2.表语从句：表语从句出现在系动词后，充当表语。

The trouble is that we are short of money.

 (1)  引导表语从句除了上述三类词外还有because, as if/ as though等等

 The reason (why/for which…..) is that ……

 It/This/That is because……

(2)  系动词“appear, look, seem”的两个常用句型

It seems/appears that….

It looks/seems as if/as though……(与事实相符用陈述语气，与事实相反用虚拟语气)

(3)  as 也可以引导表语从句

 Things are not always as they seem to be

       3. 宾语从句：及物动词，形容词和介词后加宾语从句作其宾语。

(1)   宾语从句时态与主句相呼应，但宾语从句表示的是客观真理或普遍现象除外

The teacher told us that the earth moves around the sun.

(2)   连词whether和 if可以互换，但注意下列情况

连词后紧跟or not时用whether

I want to know whether or not they will come.

作介词宾语时用whether引导宾语从句

He was interested in whether the baby can stand by itself.

连接词后直接加不定式，不能用if只能用whether

He doesn’t know whether to stay or not.

如果宾语从句是否定时，一般用if引导

I care if he will not attend the meeting.

(3)   当宾语从句后带宾补时，要用“主语+谓语+it +宾补+that-clause, that不可省略

I think it certain that she will do well in her exam.

(4)   当主语是I, we，主句谓语是think, believe, suppose, expect, imagine等动词时，用否定前移

I don’t think he will win the game, will he?

(5)   that在宾语从句常可以省略，但由and或 but连接两个或多个宾语从句时，仅可以省略第一个连词that

He said (that) he had eaten nothing but that he wasn’t hungry.

(6)   注意区别if引导的宾语从句和条件状语从句

I don’t know if he will come.

If he comes, I’ll let you know.

       4. 名词性从句注意事项

1）疑问词-ever既可以引导名词性从句，也可以引导让步状语从句，而no matter+疑问词只能引导让步状语从句

I believe whatever he says. （宾语从句） 

Whatever he says, I will never believe him. （让步状语从句）

No matter where I go, my heart is towards China.（让步状语从句）

×I believe no matter what he says. (错句)

who与 whoever的区别

who是“谁”的意思，表示具有疑问，whoever相当于anyone who“无论谁”

Whoever leaves the room last ought to turn off the lights.

Can you tell me who that gentleman is?

2）．当名词性从句缺少主语、宾语或表语时，引导词通常用what，它相当于all that, anything that或 the thing(s) that。另外，有时具有感叹意义

What we can’t get seems better than what we have.

Tom thought what a beautiful girl Mary was.

3）.注意区分it 作形式主语的主语从句与强调句

强调句句型It is/was+被强调部分+that, 去掉It is/was, that，句子仍然成立。

It is natural that they should have different views. 主语从句

It is only lately that he had had a family himself. 强调句

What was it that he wanted?   I don’t know what it was that he wanted.

It was in the hotel where he stayed that we discussed the serious problem.

4）.注意it 作形式主语的主语从句和as引导非限制性定语从句引导的不同

It is known to all that the earth is round.

As is known to all, the earth is round.