史上最全的主语从句讲解及其练习
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主语从句
一、主语从句的连词分三类
（1）从属连词引导的主语从句（that whether if）
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。That 引导主语从句通常用it 作形式主语。
That she survived the accident is a miracle.
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不能在开头，只能用whether。
Whether she is coming or not doesn’t matter too much.
（2）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Whatever we do is to serve the people.
注：whatever / whoever的功用
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. ）
（3）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、it 作形式主语的主语从句
有时为了考虑句子平衡，通常在主语从句处使用形式主语it，而将真正的主语从句移至句末。这分四种情况：
(1)对于以连词that引导的主语从句，通常用形式主语it替代主语从句：
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。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 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.
It is doubtful whether she will be able to come.
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.
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。
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 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 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.
(2)对于以连接代词(副词)引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句：
Whether they would support us was a problem. 他们是否会支持我们还是一个问题。
It was a problem whether they would support us.他们是否会支持我们还是一个问题。
(3)对关系代词型what引导的主语从句，通常直接将主语从句放在句首。如：