一、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。
1. It 作形式主语和it引导强调句的比较：
It 作形式主语代替主语从句，主要是为了平衡句子结构，主语从句的连接词没有变化。而it引导的强调句则是对句子某一部分进行强调，无论强调的是什么成分，都可用连词that。被强调部分指人是也可用who/whom。例如：
It is a pity that you didn’t go to see the film.
你没有去看这个电影，多可惜啊！
It doesn’t interest me whether you succeed or not.
对于你是否成功，我并不感兴趣。
It is in the morning that the murder took place.
凶杀案发生的时间是在早上。
It is John that broke the window.
打碎窗户的是约翰。
2.用it 作形式主语的结构
(1)It is ＋名词＋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
It’s a pity that you missed the exciting football match.
多么可惜啊，你错过了一场精彩的足球比赛。
(2)it is ＋形容词＋从句
It is natural that… 很自然… It is strange that… 奇怪的是…
It is uncertain whether he can come to Jenny’s birthday party or not.
他来不来参加简的生日派对，这个不确定。
(3)it is ＋不及物动词＋从句
It seems that… 似乎… It happened that… 碰巧…
It seemed that he didn’t tell the truth.
似乎他没有讲真话。
It happened that I was out when he called.
碰巧，他打电话的时候我在外面。
(4)it ＋过去分词＋从句
It is reported that… 据报道… It has been proved that… 已证实…
It is reported that 16 people were killed in the earthquake.
据报道，在这场地震中，有16人遇难。