在英语中有一类句型叫做名词性从句，它又包括以下句型：主语从句，表语从句，宾语从句，同位语从句，今天我们先来了解下主语从句：
一、定义
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
e.g.（What it was to become） was uncertain 。
它将变成什么样是不确定的。
what引导的从句what it was to become，在整个句子中作主语，所以what引导的这个从句叫做主语从句。
二、引导词
连词 that , whether
e.g. That she is still alive is a consolation.她仍然活着，这让人很欣慰。
连接代词 who(ever), whom(ever), which(ever), what(ever)
e.g.Whoever breaks the law should be punished。无论谁触犯了法律都应该受到惩罚。
连接副词 where ,when ,why ,how
e.g.Where he came from still puzzles all of us.他来自哪里仍然对我们来说是个谜。
三、形式主语it
有时为了考虑句子平衡, 通常在含有主语从句的复合句中使用形式主语it, 而将真正的主语从句移至句末。常见的用形式主语it的主语从句结构有:
1) It is + n（名词） +从句
It is a pity/shame that... 遗憾的是…… It is a surprise that…令人惊奇的是… It is a fact that… …是事实 It is common knowledge that… …是常识
2) It is + adj（形容词） +从句
It’s certain that… 肯定… It is possible that... 很可能…… It is unlikely that... 不可能…… It is obvious that… 很明显… It is necessary \important \natural\... that…
3) It +特殊动词+从句
It happened that... 碰巧…… It occurred to me that…我突然想起…
4) It + be +过去分词+从句
It is said/known/reported/believed that... 据说…/众所周知…/据报道…/据信…; It is suggested that + sb. (should）do: 建议… It must be admitted that…必须承认…… It cannot be denied that… 不可否认…… It must be pointed out that…需指出的是…
四、练习
1. _____ we need more practice is quite
obvious.
A. What B. That C. When D. /
2. _______ they can solve the question is still
unknown.
A. If B. Which C. Whether D. What
5. ____ wants to stay in a hotel has to pay their own way.
A. Anyone B. The one
C. Whoever D. Who