主語從句是在復合句中充當主語的從句，通常放在主句謂語動詞之前或由形式主語it代替，而本身放在句子末尾。
1. It 作形式主語和it引導強調句的比較
　 It 作形式主語代替主語從句，主要是為了平衡句子結構，主語從句的連接詞沒有變化。而it引導的強調句則是對句子某一部分進行強調，無論強調的是什么成分，都可用連詞that。被強調部分指人時也可用who/whom。例如：
a) It is a pity that you didn’t go to see the film. 你不去看那場電影真可惜。
　　b) It doesn’t interest me whether you succeed or not.　我對你成功與否不感興趣。
　　c) It is in the morning that the murder took place.　謀殺案是在早上發(fā)生的。(強調句型)
　　d) It is John that broke the window.　是John打碎的窗戶。(強調句型)

2. 用it 作形式主語的結構
　 (1) It is + 名詞 + 從句
　　It is a fact that … 事實是…
　　It is an honor that …非常榮幸
　　It is common knowledge that …是常識
(2) It is + 形容詞 + 從句
　　It is natural that… 很自然…
　　It is strange that… 奇怪的是…
　 (3) It is + 不及物動詞 + 從句
　　It seems that… 似乎…
　　It happened that… 碰巧…
　　It appears that… 似乎…
(4) It + 過去分詞 + 從句
　　It is reported that… 據(jù)報道…
　　It has been proved that… 已證實…
　　It is said that… 據(jù)說…
3. 主語從句不可位于句首的五種情況：
　 (1)if 引導的主語從句不可居于復合句句首。
　 (2)It is said /reported…結構中的主語從句不可提前。例如：
　　正確表達：It is said that President Jiang will visit our school next week.
　　錯誤表達：That President Jiang will visit our school next week is said.
(3)It happens/occurs…結構中的主語從句不可提前。例如：
　　正確表達：It occurred to him that he failed in the examination.
　　錯誤表達：That he failed in the examination occurred to him.
　 (4)It doesn’t matter how/whether …結構中的主語從句不可提前。例如：
　　正確表達：It doesn’t matter whether he is wrong or not.
　　錯誤表達：Whether he is wrong or not doesn’t matter.
（5)含主語從句的復合句是疑問句時，主語從句不可提前。例如：
　　正確表達：Is it likely that it will rain in the evening?
　　錯誤表達：Is that will rain in the evening likely?　
4. what 與that 在引導主語從句時的區(qū)別
　　what 引導主語從句時在句時在從句中充當句子成分，如主語.賓語.表語，而that 則不然。例如：
　　a) What you said yesterday is right.
　　b) That she is still alive is a consolation