初中英语知识点：主语从句
在复合句中充当主语成分的句子叫做主语从句。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。
例如：
That he finished writing the composition in such a short time surprised us all.
What we need is time.
It is certain that he will win the match.
(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第三人称单数
主语从句引导词：
1、由连词that引导的主语从句。
例如：That you will win the medal seems unlikely.
你想获得奖牌看起来是不可能的。
That you are so indifferent bothers me.
你如此冷淡使我很烦恼。
That she survived the accident is a miracle.
她在事故中幸免于难简直是奇迹。
2、用连接代词或连接副词if, whether 引导的主语从句。
例如： Which of them escaped from the prison is still a mystery.
他们哪一个是从监狱里逃跑的，仍然是个谜。
When they will come hasn’t been made pubic.
他们什么时候来还不知道。
Whether she is coming or not doesn’t matter too much.
她来不来都无关紧要。
3、用关系代词引导的主语从句。
例如：What you need is more practice.
你所需要的是更多的训练。
What I want to know is this. 我想知道的就是这事。
Whatever we do is to serve the people.
我们无论做什么都是为人民服务。
4、主语从句可以直接放在主语位置上，也可以用it作形式主语，而将从句放在句末(尤其是当谓语较短时)。
例如：That light travels in straight lines is known to all.
光沿直线传播，这是众所周知的事。
=It is known to all that light travels in straight lines.
众所周知光沿直线传播。
When the plane is to take off has not been announced .
飞机何时起飞还没有宣布。
= It has not been announced when the plane is to take off.
还没有宣布飞机何时起飞。
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. 他想要的是本书。
如果带主语从句的句子是疑问式，则必须用it作形式主语的结构。
例如：Has it been settled who will be sent to carry out the task ?
谁将被派去执行这项任务决定了吗 ?
主语从句与宾语从句区别：
主语从句与宾语从句两者的用法和在句中的作用是不同的，下面从它们的各自用法来介绍。
一.主语从句
主语从句 定义：在句子中担当主语的是一个从句，这个从句就叫做主语从句。
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
1. 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。
初中实用英语语法：主语从句学习指要
用来作主语的句子叫做主语从句。如：
(1)When he was born is unknown. 他生于何时还不知道。
(2)What he did last night is being investigated. 他昨天晚上干了些什么正在调查之中。
在主语从句中须注意：
1. 主语从句一般用it作形式主语，放在句首，而将主语从句放到句末。如：
(1)It is possible that he has stolen the car. 很可能他偷了小车。
(2)Isn’t it strange that he should not have passed the test? 他测试没有通过，难道不奇怪吗?
2. if引导主语从句时，只能用it作形式主语，放在句首，而将if引导的主语从句放到句末。如：
It is uncertain if he will leave for Beijing tomorrow. 明天他是否去北京还不敢肯定。
3. that引导主语从句放在句首时，that不可省，但在句末口语中可省。如：
That he was ill yesterday is known now. 大家都已知道他昨天生了病。
4. 连接代词引导主语从句只能放在句首。如;
(1)Who has broken the glass is unknown. 谁打破了玻璃现在还不知道。
(2)What he wants is all here. 他所要的东西都在这里。
5. 主语从句中主句的谓语动词一般用第三人称单数形式，但what引导主语从句的主句谓语动词的形式须根据句子的意思来判断。如：
(1)How he worked it out is still a secret. 他是如何把它做出来的还是一个秘密
(2)What they makes in this factory are TV sets. 他们在工厂里所做的东西就是电视机。
英语语法：主语从句全面剖析
一、主语从句的引导词
主语从句通常由连词that和whether、连接代词或连接副词以及关系代词型what引导。
1. that引导
That she is still alive is a consolation. 她还活着是使人感到宽慰的。
That she became an artist may have been due to her father's influence. 她成为画家可能是受她父亲的影响。
That you are coming to London is the best news I have heard this long time. 你将来伦敦是好久以来我听到的消息。
That she was chosen made a tremendous stir in her village. 她被挑选上，在她村子里引起很大轰动。
2. whether引导
Whether it will do us harm remains to be seen. 是否对我们有害还要看一看。
Whether they would support us was a problem. 他们是否会支持我们还是一个问题。
3. 连接代词引导
Whoever is top from wins the game when two matched players meet. 两强相争勇者胜。
Whichever (of you) comes in first will receive a prize. 无论(你们)谁先到都可以得奖。
Whichever you want is yours. 你要哪个哪个就是你的。
4. 连接副词引导
When we arrive doesn't matter. 什么时候到没有关系。
How it was done was a mystery. 这是怎样做的是一个谜。
Where I spend my summer is no business of yours. 我在哪里过暑假不关你的事。
5. 关系代词型what引导
What we need is money. 我们需要的是钱。
What I want to know is this. 我想知道的就是这一点。
What's done is done. 事已成定局。
What he says is not important. 他说的话并不重要。
What I am telling you is mere impressions. 我和你说的都不过是一些印象而已。
What make the river more beautiful are the lotus plants growing in the water. 为河水增色的是水里的荷花。
What you need is a good-sized canvas bag. 你需要的是一个比较大的帆布袋。
What I am afraid of is their taking him to Portugal. 我担心的是他们把他带到葡萄牙去。
What struck me was that they have all suffered a lot. 给我很深印象的是他们都受过很多苦。
二、主语从句与形式主语it
(1) 对于以连词that引导的主语从句，通常用形式主语代主语从句：
It's a pity that he didn't come. 很遗憾他没来。
It is important that he should know about this. 他必须知道此事。
It's vital that we be present. 我们出席是至关重要的。
It was intended that you be the candidate. 大家的意图是让你当候选人。
It is important that this mission not fail. 这项使命不失败至关重要。
It is essential that a meeting be convened this week. 本周开一次会非常重要。
It is appropriate that this tax be abolished. 废除这个税是恰当的。
It's unfair that so many people should lose their jobs. 竟有这么多人失业这是不公平的。
(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。
It remains to be seen whether it will do us harm or good. 它对我们是利是害，还得看看再说。
(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：
What I want to know is this. 我想知道的就是这一点。
有时也可将助用形式主语。如：
It is clear enough what he meant. 他是什么意思很清楚。
(4) 如果句子是疑问句，则必须用带形式主语it的结构：
How is it that you are late again? 你怎么又迟到了?