什么是主语从句?

主语从句，即在复合句中充当主语成分的句子。

例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。

用法

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。

It 作形式主语和it引导强调句的比较

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.

在句子中担当主语的从句就叫做主语从句，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。

什么是主语从句，最简单直白的解释就是在复合句中作主语成分的句子。
不可不知的主语从句

以下文段摘自2017年高考全国卷英语试题阅读第3篇：
Joy高斋翻译CATTI和MTI分享：
Some of the world’s most famous musicians recently gathered in Paris and New Orleans to celebrate the first annual International Jazz Day. UNESCO( United Nations Educational, Scientific and Cultural Organization) recently set April 30 as a day to raise awareness of jazz music, its significance, and its potential as a unifying(联合) voice across cultures.
Despite the celebrations, though, in the U.S. the jazz audience continues to shrink and grow older, and the music has failed to connect with younger generations.
It’s Jason Moran’s job to help change that. As the Kennedy Center’s artistic adviser for jazz, Moran hopes to widen the audience for jazz, make the music more accessible, and preserve its history and culture.
“Jazz seems like it’s not really a part of the American appetite,” Moran tells National Public Radio’s reporter Neal Conan. “What I’m hoping to accomplish is that my generation and younger start to reconsider and understand that jazz is not black and white anymore. It’s actually color, and it’s actually digital.”
这篇文章以欢庆第一届“国际爵士乐日”（ International Jazz Day）引出下文，指出这个节日的创立能够帮助人们加深对爵士乐的认识，认识它的重要意义，并提出它能促进不同文化间的交流融合（to raise awareness of jazz music, its significance, and its potential as a unifying voice across cultures）。
但是，接下来文章以一个“though”字转折了一下，指出尽管有这个庆祝节日，美国欣赏爵士乐的人还是越来越少，而且大多是老人，年轻一代并不喜欢爵士乐（the music has failed to connect with younger generations）。
因此，Jason Moran希望能改变这个状况。
他表示，“What I’m hoping to accomplish is that my generation and younger start to reconsider and understand that jazz is not black and white anymore. It’s actually color, and it’s actually digital.” 这句话实际上包含一个主语从句。我们将通过这个例子来总结一下主语从句的用法。
What I’m hoping to accomplish is that my generation and younger start to reconsider and understand that jazz is not black and white anymore
这句话的主语是一个小从句，“what i am hoping to accomplish ”，即我希望能实现什么。后面的“that my generation and younger start to … anymore” 是what的具体内容，what作宾语，即to accomplish what。
因此，整句话的意思就是说我希望我这一代人，包括年轻人在内，都能重新思考，明白爵士不再是黑人或白人的专利。
主语从句，顾名思义，就是说在句子中担当主语的是一个从句。
第一，像what引导的主语从句，在句子中what是要做成分的。
比如说What you said yesterday is right. 你昨天说的是对的。what作宾语，即said what yesterday。
第二，从句的位置？
1、它通常放在主句的谓语动词之前，比如说这篇文章里的这句话，主句的谓语动词是is，从句what i am hoping to accomplish放在is的前面。
2、或者用形式主语it代替。真正的主语则放在句子末尾。如果不是从句不是特别长，也可以直接放在句首。
举两个例子。
A. It is still a question whether she will come or not. 她来不来还是个问题。
这句话实际的主语是whether she will come or not，将句子还原一下，就变成了whether she will come or not is still a question。
B. It is still unknown which team will win the match. 哪一支队伍将赢得比赛还是个未知数，
同理，这句话的真正主语是which team will win the match，实际上是which team will win the match is still unknown。
第三，还有以下几种特殊情况，我们不能使用1的句式，不能把主语从句放在句首，位于谓语动词之前，这些情况下真正的主语都放在句子后面。下面这些也是常见固定句型。
A. It is said (reported) that 据说，据报道，……。
比如，it is said that the president will visit our school next week.
据说总统下周要来访我校。
这句话真正的主语是the president will visit our school next week，但是在这个句式中，我们不能写成the president will visit our school next week is said/reported。
B. It happens (occurs)
1）It occurred to somebody；
比如，It occurred to him that he failed in the exam. 他想到他考试挂了。
（he failed in the exam occurred to him） ×
occur to somebody是一个固定表达，意思是（观念、想法或主意）出现在头脑中，什么什么被想起。
It didn’t occur to her to ask for help. 就是说，她没有想到请别人帮忙。
（To ask for help did not occur to her） ×
2）It happens that，意思是发生、出现、碰巧……。
It happens that I am not there that day. 碰巧那天我不在那儿。
It would happen that the software was written in a week instead of three months.
偏偏这套软件就在一周内被编写出来，而不是三个月。
Tip: 有一点需要注意，像The idea occurred to him in a dream（这个主意是他在梦中想到的）这样的句子就不是主语从句了，这句话的主语就是the idea。
C. It doesn’t matter how/whether/if 意思是怎样，是否……都没关系，不重要。
It doesn’t matter whether he comes or not. 他来不来都没关系。
（whether he comes or not doesn’t matter）×
It doesn't matter how you do it. 你怎么做都没关系。
（how you do it doesn’t matter） ×
D. 疑问句中。
Is it likely that it will rain in the evening? 傍晚有可能下雨吗？It作形式主语，真正的主语是that it will rain in the evening。
How does it happen that you know her? 你怎么会碰巧认识她的？真正的主语是that you know her。
最后总结一下今天所说的，
一、介绍了主语从句中从句的位置。一般有两种形式。
二、介绍了四中特殊情况下主语从句不能放在句首，当然，这部分也可以当作固定表达来记忆。

　　主语从句即在复合句中充当主语成分的句子。
　　例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。
　　引导主语从句的连词主要有：
　　从属代词：that whether
　　连接代词：who whoever whom whose what whatever which whichever
　　连接副词：when where how why

主语从句

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、 几个共性问题：
1．连接方式
（1） 在由连词that, whether, if引导的名词性从句中，引导词在句中不做成分，在宾语从句和表语从句中，that可以省略。
（2） 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。
（3）在由连接副词when, where, why, how引导的名词性从句中，其连接副词在句中作状语。
2．whether和if的区别
（1）whether在引导的主语从句中可以置于句首，而if则不能。
（2）whether可以与or not连用，or not可以紧跟在whether之后，也可以置于从句末尾；if一般不能和or not连用。
（3）表语从句、同位语从句和介词宾语从句（即在系词be和介词之后），只能用whether连接，而不用if。
（4）从句是否定式时，一般用if, whether很少用在否定式从句中。
例如：I don’t care if he doesn’t come . 如果他不来，我也不介意。
The problem is whether we can carry out the plan.
问题是我们是否能实施这项计划。

三、 主语从句
1．主语从句主要有三类：
（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。
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 ？
3．固定用法和译法
（1） 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.
这一切是怎么发生的是个谜。
It is common knowledge that the whale is not a fish.
鲸鱼不是鱼，这是常识。
It is no surprise that Bob should have won the game.
鲍勃会赢得这场比赛，这不足为奇。
（2） 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 obvious that conductors and insulators are both important in industry.
很明显导体和绝缘体在工业中都很重要。
It is doubtful whether she will be able to come.
她是否能来令人怀疑。
It was really astonishing that he refused to talk to you .
他拒绝和你说话真是令人惊讶。
It is essential that he should be here by the weekend .
周末之前，他应该到这里是必要的。
It seems obvious that we can not go on like this.
很明显我们不能这样下去了。
（3） 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.
大家都认为他是最好的选手。
It is estimated that the vase is 2000 years old.
据估计这个花瓶有2000年的历史。
It is used to be thought that a new star must be due to a collision between two stars. 过去一直认为新星是由于两颗星星之间碰撞产生的。
It has not been made clear when the new road is to be opened to traffic.
还没弄清楚这条路将开始通车。
（4） It +不及物动词+从句
It seems that … 好像是……
It happened that… 碰巧……
It follows that … 由此可见……
It has turned out that … 结果是……
类似的不及物动词还有：seem, appear, occur, follow, happen, turn out, etc.
例如：It turned out that nobody remembered the address.
结果是无人记得那个地址。
It now appears that they are in urgent need of help.
看起来他们急需帮助。
It does not matter if I missed my train, because there is another later.
没有赶上这趟火车没有关系，稍晚还有一趟。
It happened that I saw him yesterday.
碰巧我昨天看见他了。
当“及物动词 + 宾语”较短时，也可用这种结构。
例如：It shocked me that Peter did not tell anybody where he was.
让我吃惊的是彼得没有告诉任何人他在哪里。
It does not interest me whether you go or not.
我对你去不去不感兴趣。
It +某些固定结构+ 主语从句。
例如：It doesn’t make too much difference （It doesn’t make any difference / It doesn’t alter the situation），whether it rains or not . 下不下雨都没有多大关系。
It does not make any difference whether it rains or not .
下不下雨没什么分别。
It does not make the least difference to me what you do.
对我来说你做什么都无关紧要。
Is it of much consequence to you that Mr. Wang has failed to do it ? 王先生没有做成这件事对你有什么重要影响吗？