在句中作主语的名词性从句称为主语从句， 主语从句所对应的谓语动词通常用第三人称单数的形式。为了整个句子的平衡， 主语从句经常后置， 并用it充当形式主语(此时， that引导的主语从句若不放在句首， 可以省略引导词that)。
一、that在句中不做成分。当主语从句位于句首时，不能省略：
That he likes books of this kind is very interesting.= It’s very interesting (that)he likes books of this kind.
这是用的最多的一种主语从句,但把这种主语从句放在句首是很少的,只是为了强调或谓语较长时才这样安排.例如：
二、当谓语动词是seem,appear,be a pity.be a wonder,be likely等时，使用形式主语It，而将真正的主语放在句尾，这时从属连词that可省略：It appeared to scientists on earth (that) the stars had moved.
还有一些例如：绝大部分主语从句都借助先行主语放到句子后部去.
1.It +be + 形容词+that从句：
It was quite obvious that she didn’t want to come.
2.It +be + 名词 +that 从句：
It’s a pity that he can’t swim.
3.It + 动词（+ 宾语或状语）+ that 从句：
It happened that she wasn’t in that day.
4.It + 动词的被动语态 + that 从句：
It’s reported that the enemy troops have crossed the border.
