定義:在復合句中充當主語的句子叫做主語從句.第一部分：常規(guī)主語從句,即句子在復合句中充當一個主語 主語從句的時態(tài)：不受主句的時態(tài)影響和限制.（1）That he finished writing the composition in such a short time surprised us all.（1)引導主語從句連詞有that,whether,who,what,whatever等 （2)連詞位于句首不能省略 （3)主語從句大多數(shù)情況下視為單三,但也有例外,如例 What we need are good doctors.第二部分：為了防止句子頭重腳輕,通常把形式主語it放在主語位置,真正主語擱置于句末 （1）It is certain that he will win the match.（1)以that 引出的主語從句,常以形式主語it引導.It is +形容詞/名詞/某些動詞ed + that 從句.（2) 在有些that從句中要用虛擬語氣 (should+do/should+have done)例句 （1）It is strange that he should do that.（2）It is important that we all should attend the meeting.（3）It is strange that the man should have stuck to his silly ideas.（4）It is suggested that the work should be done with great care.主語從句與賓語從句兩者的用法和在句中的作用是不同的,下面從它們的各自用法,介紹：一．主語從句 主語從句 定義:在句子中擔當主語的是一個從句,這個從句就叫做主語從句.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(diào)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(jié)構(gòu),主語從句的連接詞沒有變化.而it引導的強調(diào)句則是對句子某一部分進行強調(diào),無論強調(diào)的是什么成分,都可用連詞that.被強調(diào)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.2.用it 作形式主語的結(jié)構(gòu) (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 ＋不及物動詞＋從句 It seems that… 似乎… It happened that… 碰巧… (4) it is＋過去 分詞 ＋從句 It is reported that… 據(jù)報道… It has been proved that… 已證實… 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(jié)構(gòu)中的主語從句不可提前.例如：It is said that President Jingo will visit our school next week.(3) It happens…,It occurs… 結(jié)構(gòu)中的主語從句不可提前.例如：It occurred to him that he failed in the examination.(4) It doesn’t matter how/whether …結(jié)構(gòu)中的主語從句不可提前.例如：It doesn’t matter whether he is wrong or not.(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：Is it likely that it will rain in the evening?4.What 與that 在引導主語從句時的區(qū)別 What 引導主語從句時在從句中充當 句子成分 ,如主語．賓語．表語,而that 則不然.例如：What you said yesterday is right.