高考主语从句考点解析，主语从句的连词主要分三类：
（1）从属连词引导的主语从句（that whether）
that引导的主语从句: that无含义，在句中不做成分，不可以省。通常用it作形式主语。
That you will win the medal seems unlikely. It is a miracle that she survived the accident.
whether 引导的主语从句: whether有含义(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） 用连接代词引导的主语从句
在由连接代词who, whose, whom, which, what, whoever, whomever, whichever, whatever引导的名词性从句中，其连接代词在句中起名词性作用，担当一定成分.
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）用连接副词when, where, why, how引导的名词性从句（有含义，在句中作状语。)
Where we should leave it is a problem. When they will come hasn’t been made public.
二、it 作形式主语的主语从句
有时为了句子平衡，通常在主语从句处使用形式主语it，而将真正的主语从句首移至句末。
(1) 对于以连词that引导的主语从句，通常用形式主语it替代主语从句：
It is +名词+从句
It is a fact that… It is good news that；It is a question that；It is common knowledge that …
类似的名词还有：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 +形容词+从句
It is necessary that …；It is clear that …；It is likely that …；It is important that …
类似的形容词还有：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 +过去分词+从句
It is said that …；It is reported that …；It has been proved that …；It must be proved that…
类似的过去分词还有：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 +不及物动词+从句（seems / happened / appears / doesn’t matter / makes no difference / occurred …）that …主语从句不可提前
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) 对于以连接代词(副词)引导的主语从句，可以使用形式主语代替主语从句，也可直接在句首使用主语从句：
Whether they would support us was a problem. It was a problem whether they would support us.
(3) 对关系代词what引导的主语从句，通常直接将主语从句放在句首: What we need is money.
