定义：如果一个句子在复合句中充当一个主语，那么这个句子就是主语从句• 第一部分：常规主语从句，即句子在复合句中充当一个主语（1）That he finished writing the composition in such a short time surprised me. （2）Whether we will go for an out ing tomorrow rema ins unknown.（3）Who will be our monitor has't been decided yet.（4）Whom we must study for is a question of great importanee.（5）What caused the accide nt rema ins unknown.（6）Whatever you did is right.（7）Whose watch was lost is unknown.（8）What we need is time.（9）What we need are good doctors.小结：（1）引导主语从句连词有that,whether,who,what,whatever 等（2）连词位于句首不能省略（3）主语从句大多数情况下视为单三，但也有例外，如例9第二部分：为了防止句子头重脚轻，通常把形式主语it放在主语位置，真正主语搁置于句末（1）It is certain that he will win the match.（2）It is true that he has madea very important discovery in chemistry.（3）It is very likely that they will hold a meeting.（4）It is strange that he should do that.（5）It is important that we all should attend the meeting.（6）It is strange that the man should have stuck to his silly ideas.（7）It is a pity that we won't be able to go to the south to spend our vacati on. （8）It is still a mystery what caused the accident.（9）It is said that he has gone to Taiwan.（=He is said to have gone toTaiwa n）（10）It is known to all that the gun powder was first inven ted by theChin ese.（11）It is suggested that the work should be done with great care.（12）It seems that he has seen the film.（二He seemsto have seen the film）（13）It happenedthat the two cheats were there. （=The two cheats happened to be there）小结：（1）以that引出的主语从句,常以形式主语it引导.It is + 形容词/名词/某些动词ed + that 从句.（2）在有些that从句中要用虚拟语气（should+do/should+have done）例句4,5,6,11.名词性从句在句子中起名词作用的句子叫名词从句（Noun Clauses ）。
名词从句的功能相当于名词词组, 它在复合句中能担任主语、宾语、表语、同位语、介词宾语等，因此根据它在句中不同的语法功能，名词从句又可分别称为主语从句、宾语从句、表语从句和同位语从句。
一、引导名词性从句的连接词引导名词性从句的连接词可分为三类：连词：that （无任何词意）whether, if 均表示“是否”表明从句内容的不确定性）as if ,as though （均表示“好像”，“似乎”）以上在从句中均不充当任何成分连接代词：what, whatever, who, whoever, whom,whose, which. whichever, whomever连接副词：when, where, how, why不可省略的连词：1. 介词后的连词2. 引导主语从句和同位语从句的连词不可省略。
That she was chosen made us very happy.We heard the news that our team had won.比较：whether 与if 均为"是否"的意思。
但在下列情况下，whether 不能被if 取 代：1. whether 引导主语从句并在句首2. 引导表语从句3. whether 从句作介词宾语4. 从句后有 "or not"Whether he will come is not clear.大部分连接词引导的主语从句都可以置于句末，用 it 充当形式主语。
It is not important who will go.It is still unknown which team will win the match.二 . 主语从句1. 作句子主语的从句叫主语从句。
主语从句通常由从属连词 that ，whether , if 和连接代词 what , who which , whatever , whoever 以及连接副词 how, when, where ，why 等词引导。
that 在句中无词义，只起连接作用；连接代 词和连接副词在句中既保留自己的疑问含义、 又起连接作用, 在从句中充当从句 的成分。
例如：What he wants to tell us is not clear.Who will win the match is still unknown. 知。
It is known to us how he became a writer.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。
他要跟我们说什么,还不清楚 谁能赢得这场比赛还不得而我们都知道他是如何成为一2. 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（2）It + be + 形容词+ that 从句（3）It + be + 动词的过去分词+ that 从句It is believed that 25 years after construction, the building was dismantled and moved timber by timber to the South Bank of the Thames, where a reconstruction of the theatre now stands.据信经过25 年的建设后, 该建筑物将被拆处. 并且该建筑物的一砖一石将被移到泰唔士河的南岸. 重建的戏剧院现在正座落于此。
It is thought the cost of this lost labour is around £ 1 billio n （10 billi on yua n）to businesses and the economy.大家一直认为这次对贸易和经济失去劳动力的损失的费用应该在10 亿英镑（人民币100 亿元）It is not known how many of the 59 million words Mr Shea has remembered but he has certainly made history with his eccentric hobby.在590 万的词汇中到底记住了多少不得为知, 但是她却以这样的怪癖创造了历史。
4）It + 不及物动词+ that 从句3. 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 n ecessary (importa nt, n atural, stra nge, etc.) that …It is a pity (a shame, no won der, etc.) that •…It is suggested (requested, proposed, desired, etc.) th at …4. 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. It doesn't interest me whether you succeed or not.It is in the morning that the murder took place. It is John that broke the window.5. What 与that 在引导主语从句时的区别What 引导主语从句时在句时在从句中充当句子成分，如主语．宾语．表语，而that 则不然。
(1) What you said yesterday is right. (2) That she is still aliveis a consolation.名词性从句--- 宾语从句; 表语从句(2009-03-18 22:15:32)转载三、宾语从句名词句用作宾语的从句叫宾语从句。
引导宾语从句的关联词与引导主语从句表语从句的关联词大致一样，在句中可以作谓语动词或介词及非谓语动词的宾语。
1. 作动词的宾语:1) . 由that 引导的宾语从句( that 通常可以省略, 例如：He has told me that he will go to Shanghai tomorrow. 他已经告诉我他明天要去上海。