名词性从句名词性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，名词性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。
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。
常规主语从句，即句子在复合句中充当一个主语主语从句的时态：不受主句的时态影响和限制。
（1）That he finished writing the composition in such a short time surprised us all.（2）Whether we will go for an outing tomorrow remains unknown.（3）Who will be our monitor hasn't been decided yet.（4）Whom we must study for is a question of great importance.（5）What caused the accident remains unknown.（6）Whatever you did is right.（7）Who the watch belongs to is unknown.（8）What we need is time.连词位于句首不能省略为避免头重脚轻，常以形式主语it引导从句。
It+形式主语谓语+从句。
由于主语从句一般被认为是单数形式，形式主语的谓语动词、be动词+表语均为单数第三人称形式或过去式.It 作形式主语和it引导强调句的比较①主语从句常用it作形式主语，一般常用句型为：It is+{名词/形容词/过去分词}+主语从句例如：It is still a question whether she will come or not.It is strange that you should like him.It is still unknown which team will win the match.另外，还有一些比较多见的结构：It turned out that……;It has been proved that……;It happened/occurred that……;It is well-known that……等等②而强调句则不同，它的结构是：It+be+被强调部分+that+从句强调主语：It is the times that produce their heroes.时势造英雄强调宾语：It is English that Prof.Lin teaches us.强调状语：It was in shanghai that I saw the film.判断是否是强调句有一个方法，就是将that以后的“句子的其余部分”拿出来单独看，看有没有缺成分，缺的是什么成分，再把谓语动词后面that前面的那部分，带到你认为缺成分的地方，如果放进去是一句完整的句子了，那就说明是强调句。
2. 用it 作形式主语的结构(1) It is +名词+that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(2) it is +形容词+that从句It is natural that…很自然……It is strange that…奇怪的是……(3) it +不及物动词+that从句It seems that…似乎……It happened that…碰巧……(4) it is+过去分词+that从句It is reported that…据报道……It has been proved that…已证实……3.主语从句不可位于句首的五种情况(1) if 引导的主语从句不可居于复合句句首。
(2) It is said ,(reported) …结构中的主语从句不可提前。
例如：It is said that President Jingo will visit our school next week. (3) It happens…, It occurs…结构中的主语从句不可提前。
例如：It occurred to him that he failed in the examination.(4) It doesn’t matter how/whether …结构中的主语从句不可提前。
例如：It doesn’t matter whether he is wrong or not.(5) 含主语从句的复合句是疑问句时，主语从句不可提前。
例如：Is it likely that it will rain in the evening?4. What 与that 在引导主语从句时的区别What 引导主语从句时在从句中充当句子成分，如主语．宾语．表语，而that 则不然。
例如：What you said yesterday is right.宾语从句有三种情况值得注意(1)在介词后面只能用whether不能用if。
例如①This depends upon whether we are determined to do it.②It depends on whether he is ready.③I am not interested in whether you'll come or not.④We haven't settled the question of whether we'll renew our supplies of coal for factory.(2)在动词不定式之前只能用whether不能用if。
例如①He doesn't know whether to stay or not.②She doesn't knows whether to get married now or wait.(3)在及物动词discuss后的宾语从句中只能用whether不能用if。
例如①We discussed whether we should make a change in our plan.②We were discussing whether we should discuss the business with them.同位语从句一、在复合句中用作同位语的从句叫同位语从句。
它一般跟在某些名词后面，用以说明该名词表示的具体内容。
二、可以跟同位语从句的名词通常有news，idea，fact，promise，question，doubt，thought，hope，message，suggestion，words（消息），possibility等（一般的“抽象”名词都可用）。
I’ve come from Mr wang with a message that he won’t be able to see you this afternoon．三、英语中引导同位语从句的词通常有连词that，whether，连接代词what，who。
连接副词how,when,where等。
（注：if,which 不能引导同位语从句。
）He must answer the question whether he agrees to it or not.四、有时同位语从句可以不紧跟在说明的名词后面，而被别的词隔开。
The thought came to him that maybe the enemy had fled the city.表语从句1.表语从句一定要用陈述语序。
False: The question is when can he arrive at the hotel.Right: The question is when he can arrive at the hotel.2.不可以用if，而用whether 连接表语从句(as if 例外）。
引导宾语从句时可以互换if/whether 位于介词后要用whether，位于句首时要用whether 引导表语从句，主语从句，同位语从句时要用whetherFalse: The question is if the enemy is marching towards us.Right: The question is whether the enemy is marching towards us.3.不像宾语从句，在有表语从句的复合句中，主句时态和从句时态可以不一致。
Right: The question is who will travel with me to Beijing tomorrow.Right: The question is why he cried yesterday.4.that在表语从句中不可以省掉。
表语从句的基本用法:表语从句只能置于主句之后，而主句的动词只能是联系动词。
名词性从句在be等系动词后作表语时被称为表语从句.例如： The problem is how we can get the things we need. (how 在表语从句中充当方式状语)问题是我们怎样能弄到我们需要的东西。
名词性从句的练习：1.Dad is used to smoking and drinking.There's no chance ________ I'm able to talk him into ________.A．whether；giving it up B．of whether；giving them upC．that；getting rid of them D．which；stopping it答案 C [由smoking and drinking可知，A、D中的代词错误。
B项翻译不通。
注：that引导的是一个同位语从句。
]2．Keep in mind ________ you want others to respect you，you must respect others first.A．that when B．that if C．if when D．when if答案 B [that引导一个宾语从句，宾语从句中又有一个if引导的条件状语从句。
] 3．Shenzhen was only a small fishing village compared to ________ it is now. A．which B．that C．what D．where答案 C [to后是宾语从句，is后缺表语，故用what。