主语从句，即在复合句中充当主语成分的句子。例如“Who gets the job will have a lot of work to do”，得到这份工作的人都会有大量的工作要做，此复合句主要描述的是某人会有大量的工作要做，某人是该复合句的主语，因此“who gets the job”就是该复合句的主语从句。
1. 引导主语从句的连词主要有:
从属连词:that whether if
连接代词:who whoever whom whose what whatever which whichever
连接副词:when where how why whenever wherever however
注意：①连词位于句首时不能省略。
That he finished writing the composition in such a short time surprised us all. (√)
He finished writing the composition in such a short time surprised us all. (×)
②if引导的主语从句不能位于句首，且if不能和or not连用
Whether we will go for an outing tomorrow remains unknown. (√)
If we will go for an outing tomorrow remains unknown. (×)
It’s still unknown whether or not we will go for an outing tomorrow. (√)
It’s still unknown if or not we will go for an outing tomorrow. (×)
2. 当主语从句过长时，为了防止句子头重脚轻，影响理解，通常把形式主语it放在句首位置，真正主语搁置于句末。
例如：That he has made a very important discovery in chemistry is true.
主语从句为“that he has made a made a very important discovery”，做主从复合句的主语， “is”为系动词，“true”为表语，此时，句子主语过长，会出现头重脚轻的现象，不利于理解，为避免这种情况，此主从复合句写成以下形式会更好。
It is true that he has made a very important discovery in chemistry.
用it做形式主语的常用句型：
(1) It is +名词+that从句
It is a fact that … 事实是……
It is an honor that …非常荣幸
It is common knowledge that …是常识
(2) it is +形容词+that从句
It is natural that… 很自然……
It is strange that… 奇怪的是……
(3) it +不及物动词+that从句
It seems that… 似乎……
It happened that… 碰巧……
(4) it is+过去分词+that从句
It is reported that… 据报道……
It has been proved that… 已证实……
