主语从句通常由下列词引导：
1)从属连词that，whether,if等;
2)连接代词what，who，which，whatever，whoever，whom 等;
3) 连接副词how，when，where，why 等。
that在句中无词义，只起连接作用;连接代词和连接副词都在句中既保留自己的疑问含义、又起连接作用，在从句中充当主语的成分。注：whom、who指人，what指物，whatever，whoever表示泛指意义。例如：
What he wants to tell us is not clear. 他要跟我们说什么，还不清楚。
Who will win the match is still unknown. 谁能赢得这场比赛还不得而知。
It is known to us how he became a writer. 我们都知道他是如何成为一名作家的。
Where the English evening will be held has not yet been announced. 英语晚会将在哪里举行，还没有宣布。
有时为避免句子头重脚轻，常用形式主语it代替主语从句作形式主语放于句首，而把主语从句置于句末。主语从句后的谓语动词一般用单数形式。常用句型如下：
(1)It be 名词 that从句
It's a great pity(that)they didn't get married.
他们没能结婚，真是令人惋惜。
It 's a good thing(that)you were insured.
你保了险，这可是件好事。
(2)It be 形容词 that从句
It's splendid that you passed your exam.
你通过考试了，真棒。
It's strange that there are no lights on.
真奇怪，没有一盏灯是开着的。
(3)It be 动词的过去分词 主语从句
It's said that he has been there many times.
据说他去过那儿很多次。
(4)It 不及物动词 that 从句[1]
另注意在主语从句中用来表示惊奇、不相信、惋惜、理应如此等语气时，谓语动词要用虚拟语气“(should) do”，常用的句型有：
It is necessary (important, natural, strange, etc.) that …
It is suggested (requested, proposed, desired, etc.) that…
(5) it seems/happens that 从句
众所周知的几种表达方式
(1)It is known to us that.
(2)As is known to us.
(3)what is known to us is that.
it引导的强调句结构：It is/was 被强调部分 that/who 句子其它部分。
eg:My father did the experiment in the lab yesterday evening.
强调主语：It was my father who did the experiment in the lab yesterday evening.
强调宾语：It was the experiment my father did in the lab yesterday evening.
强调时间：It was yesterday evening that my father did the experiment in the lab.(注意不用when)
强调地点：It was in the lab that my father did the experiment yesterday evening.
