（1）從屬連詞引導(dǎo)的主語從句（that  whether）

that引導(dǎo)的主語從句: that無含義，在句中不做成分，不可以省。通常用it作形式主語。

That you will win the medal seems unlikely.        It is a miracle that she survived the accident.

whether 引導(dǎo)的主語從句: whether有含義(是否)，在句中不做成分/不可以省。不能用if。

Whether we will hold a party in the open air tomorrow depends on the weather.

Whether she is coming or not doesn’t matter too much.

（2） 用連接代詞引導(dǎo)的主語從句

在由連接代詞who, whose, whom, which, what, whoever, whomever, whichever, whatever引導(dǎo)的名詞性從句中，其連接代詞在句中起名詞性作用，擔(dān)當(dāng)一定成分.

What you need is more practice.          Whatever we do is to serve the people.

注：whatever / whoever的用法

whatever = anything that; whoever = anyone who

Whoever breaks the law should be punished.  ＝ Anyone who breaks the law should be punished.

（3）用連接副詞when, where, why, how引導(dǎo)的名詞性從句（有含義，在句中作狀語。)

Where we should leave it is a problem.     When they will come hasn’t been made public.

二、it 作形式主語的主語從句

有時(shí)為了句子平衡，通常在主語從句處使用形式主語it，而將真正的主語從句首移至句末。

(1) 對于以連詞that引導(dǎo)的主語從句，通常用形式主語it替代主語從句：

It is +名詞+從句

It is a fact that… It is good news that；It is a question that；It is common knowledge that …

類似的名詞還有：a pity；a wonder；a good thing； no wonder； surprise etc.

It is a mystery to me how it all happened.

It is common knowledge that the whale is not a fish.

It is +形容詞+從句

It is necessary that …；It is clear that …；It is likely that …；It is important that …

類似的形容詞還有：strange; natural; obvious; true; good; wonderful; possible; unlikely; unusual; certain; evident; worth-while; surprising; interesting; astonishing, etc.

It is doubtful whether she will be able to come. 

It is +過去分詞+從句

It is said that …；It is reported that …；It has been proved that …；It must be proved that…

類似的過去分詞還有：known; estimated; expected; believed; thought; hoped; noted; discussed; required; decided; suggested; demanded; made clear; found out, etc.

It is used to be thought that a new star must be due to a collision between two stars.

It has not been made clear when the new road is to be opened to traffic.

It +不及物動(dòng)詞+從句（seems / happened / appears / doesn’t matter / makes no difference / occurred …）that …主語從句不可提前

It makes no difference whether he will attend the meeting or not.

It does not matter if I missed my train.  Ithappened that I saw him yesterday.

(2) 對于以連接代詞(副詞)引導(dǎo)的主語從句，可以使用形式主語代替主語從句，也可直接在句首使用主語從句：

Whether they would support us was a problem. It was a problem whether they would support us.

(3) 對關(guān)系代詞what引導(dǎo)的主語從句，通常直接將主語從句放在句首: What we need is money.