一、概述：在句子中起名词作用的各种从句统称为名词性从句。根据它们在句中的语法作用，这类从句又可分为主语从句、同位语从句、表语从句和宾语从句。 

二、主语从句主要有四类：

（1） 由连词that引导的主语从句:引导词that无含义/在句中不做成分/不可以省。

例如：That you will win the medal seems unlikely.
               That you are so indifferent bothers me.
               That she survived the accident is a miracle.

（2） 用连词 whether 引导的主语从句:  whether有含义(是否)/在句中不做成分/不可以省。

注意：引导主语从句，不能用if，只能用whether。

例如：Whether we will hold a party in the open air tomorrow depends on the weather.

Whether she is coming or not doesn’t matter too much.

（3） 用连接代词引导的主语从句

在由连接代词who, whose, whom, which, what, whoever, whomever, whichever, whatever引导的名词性从句中，其连接代词在句中起名词性作用，担当一定成分.

例如：What you need is more practice.
      What I want to know is this.

Whatever we do is to serve the people.

注：whatever / whoever的功用

whatever, whoever在主语从句中不含疑问意义。它引导主语从句，并在从句中作主语、宾语、表语等。

whatever = anything that; whoever = anyone who。要注意和whatever, whoever引导的让步状语从句的区别。

如： Whoever breaks the law should be punished. （主语从句）

（ ＝Anyone who breaks the law should be punished. ）

    Whoever breaks the law, he should be punished. （让步状语从句）

（ ＝No matter who breaks the law, he should be punished. ）

（4）用连接副词when, where, why, how引导的名词性从句(其连接副词有含义,在句中作状语。)

例如:Where we should leave it is a problem.

When they will come hasn’t been made pubic.

三、由it构成的主语从句

（1）由连词that引导的主语从句，在大多数情况下会放到句子的后面，而用代词it作形式主语。

例如：That light travels in straight lines is known to all.
     =It is known to all that light travels in straight lines.
     When the plane is to take off has not been announced .
    = It has not been announced when the plane is to take off.

(但当what引导的主语从句表示“…的东西”时，一般不用it作形式主语。)

错:It is a book what he wants.

对:What he wants is a book. 

另外，需要注意的是，it作形式主语代替主语从句时，要注意和as引导的定语从句的区别。试比较：

    It was reported that the US was under the terrorist attack.

    As was reported, the US was under the terrorist attack.

（2）常见用it作形式主语的复合句结构

 <a> It is +名词+从句

It is a fact that… 事实是……

It is good news that … ……是好消息

It is a question that … ……是个问题

It is common knowledge that … ……是常识

类似的名词还有：a pity；a wonder；a good thing； no wonder； surprise等。

例如：It is a mystery to me how it all happened.

It is common knowledge that the whale is not a fish.

It is no surprise that Bob should have won the game.

It’s a pity that you missed the film.

 <b> It is +形容词+从句

It is necessary that … 有必要……

It is clear that … 很清楚……

It is likely that … 很可能……

It is important that … 重要的是……

类似的形容词还有：strange; natural; obvious; true; good; wonderful; possible; unlikely; quite;

unusual; certain; evident; worth-while; surprising; interesting; astonishing, etc.

例如：

It is doubtful whether she will be able to come.

It was really astonishing that he refused to talk to you.

It is essential that he should be here by the weekend.
      It seems obvious that we can not go on like this.

It is necessary that you （should）master the computer.

It is important that a student learn English well.

      It’s clear that they badly need help.

      It is likely that a hurricane will arrive soon.

需要注意的是，这类主语从句中，谓语动词很多为“（should）＋动词原形”，即要用虚拟语气。

<c> It is +过去分词+从句

It is said that … 据说……

It is reported that … 据报道……

It has been proved that … 已证明……

It must be proved that… 必须指出……

类似的过去分词还有：known; estimated; expected; believed; thought; hoped; noted; discussed; required; decided; suggested; demanded; made clear; found out,etc.

例如：It is thought that he is the best player.
      It is estimated that the vase is 2000 years old.
      It is used to be thought that a new star must be due to a collision between two stars.

It has not been made clear when the new road is to be opened to traffic.
      It is said that he was killed in the earthquake.

<d> It seems （happened / appears / doesn’t matter / makes no difference / occurred …）that …

如：

    It seems that they will win the game.

    It makes no difference whether he will attend the meeting or not.

    It does not matter if I missed my train.

    It happened that I saw him yesterday.