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。
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.
宾语从句用以区分主语从句的几个特征：
1、引导词：what\which\whose\when\whether\if\where
2、语序：宾语从句必须是用陈述语句。
如：I think that you must work harder.
宾语从句的引导词、连接词的区别、否定转移等现象。
(1)表达时间的几个句型：一般用when或者是what time：
What time will the train leave?
由于时刻都是固定的，也可以用一般现在时代替一般将来时：
What time does the train leave?
(2)时间的表达方式：8：19(nineteen past eight),7:57(three to eight)，在时间的中间也可以加上分的单词(minutes)
(3)had better +动词原型。意思是提要求，建议。但是上下级不能用这种方式，因为语气太重：
You had better give up smoking .
(4)sb leave sth +地点
I left my book in my classroom yesterday.
(5)may I have your name(address\age)?一般询问对方的年龄、名字等，不要习惯以前的问法，这样很不礼貌，而是要采访上面的问法。
以前的习惯是：how old are you? \what's your name?
(6)修饰名词的代词次序：限-描-大-形---年---颜----籍----物--------类+名词：
this is a bridge.
This is a beautiful bridge
补充：从句的语序永远是陈述句。
主语从句结构：
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？
相关初中英语知识点：表语从句
表语从句：
就是用一个句子作为表语。说明主语是什么或者怎么样，由名词、形容词或相当于名词或形容词的词或短语充当，和连系动词一起构成谓语。
表语从句是名词性从句的一种。
表语从句的基本结构是：
“主语+连系动词+关联词+表语从句”。
连接表语从句的连接词有：
that, what, who, when, where, which, why, whether, how, whoever，whomever，whichever ，whatever等。
例如：The question is when he can arrive at the hotel.
His suggestion is that we should stay calm.
①语序：
从句的语序必须是陈述句语序，即“主语+谓语”这种形式。
②时态：
当主句是一般现在时或一般将来时的时候，从句可以是任何时态，而当主句是一般过去时的时候，从句，从句时态必须是过去时范围的时态，即（一般过去时，过去进行时，过去完成时，过去将来时）。
③连接词：
当从句意思完整，主句意思肯定时，连接词用that，且可以省去，当从句意思完整，主句意思不确定或含否定含意时，常用if或whether（是否），当从句意思不完整时，连接词则是代替不完整部分的特殊疑问词。
表语从句种类：
根据关联词来区分,表语从句可以分为六种:
一、由that引导的表语从句
The trouble is that he misunderstood me.麻烦的是他误解了我。
有时，引导表语从句的连词that可以省略
The truth is (that) I didn’t finish my term essay. 事实是我没有写完学期论文。
二、由连接代(副)词引导的表语从句
连接代词who, whom, whose, what, which, whoever, whatever, whichever；
连接副词where, when, how, why。
That was what she said this morning in her bedroom.那就是她今天早晨在卧室中说的。
三、由关系代词型的what引导的从句
I want to be a teacher. That’s what I want to be.
我想当老师，那是我的志愿。
四、关联词是连词because
比较下面这两个句子:
I was angry. That was because he didn’t understand me.
我生气是因为他不理解我。(表语从句强调原因)
That’s why he got fired from that firm.那正是他被公司解雇的原因。(表语从句强调结果)
注意：reason作主语或主语中包含事件的起因时,后面的表语从句表示原因时要用that来引导,而不用because
That is why ...是常用句型,其中why引导的从句在句中作表语,该句型通常用于针对前面已经说过的原因进行总结。
1. That is the reason why ...That is the reason why ...与That is why ...是同义的，
但是从语法结构上讲，That is the reason why ...中的why引导的是一个定语从句，将其中的the reason去掉后则与That is why ...结构一样了
2. That is because ...That is because ...也是一个常用句型。
That is because ...句型中，连词because引导的从句作表语，
That is because ...与That is why ...之间的不同在于That is because ...指原因或理由，
That is why ...则指由于各种原因造成的结果
He did not read Harry Potter last night. That is because he had to help his mother to do the housework.
昨天晚上他没有读《哈里波特》，那是因为他得帮助他妈妈做家务。
五、关联词是从属连词whether, as, as if
The question remains whether we can win the majority of the people.
问题是我们能否赢得大多数民众的支持。
六. 使用虚拟语气的表语从句
在表示建议、劝告、命令含义的名词后的表语从句，谓语动词需用“should+动词原形” 表示，should可省略。
常见的词有：advice，suggestion，order，proposal，plan，idea等。
My suggestion is that we （should） start early tomorrow.
我的建议是我们明天一早就出发。
表语从句中需要注意的地方:
1. 表语从句一定要用陈述语序；
2. 不可以用if,而用whether 连接表语从句(as if 例外)；
3. 与宾语从句不同,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。
4. 连接词that一般不能省略，但当主句中含动词do的某种形式时，that可以省略。如：
What I want to do is (that) I can go up to him and thank him. 我想做的事是走到身边去感谢他。
5. 当句子的主语是：advice, demand, decision, desire, idea, insistence, instruction, order, plan, proposal, recommendation, request, requirement, suggestion, wish等名词时，
表语从句须用虚拟语气，即：should +do。如：
My advice is that you should go to apologize to him. 我的建议是你应该去向他道歉。
表语从句语法提示：
应注意的问题：
1. 连系动词be，appear，seem，look等之后可以跟表语从句。
It was because he didn't pass the exam.那是因为他没有通过考试。
2. 引导表语从句的引导词有以下几类。
1）wh-疑问词
My question is who left.我想问的是谁离开了。
2）whether
My question is whether he left（or not）。 我的问题是他是否离开了。
注：if不能引导表语从句。
3）that
The fact is that he left. 事实是他离开了。
注：引导表语从句的连词that间或可以省略。
The truth is（that）I didn't go there. 事实是我没去那儿。
4）because，as，as if，as though
It's just because he doesn't know her. 这是仅仅因为他不认识她。
3.表语从句要用陈述语序
That is where the famous scientist was born.
4.that和what在引导表语从句时，作用和意义都不相同。
That本身没有词义，在句中只起连接作用，不充当句子成分；what则表示“所……的（人或事）”，在表语从句中充当主语或宾语等。
That fact is that more than seventy percent of the earth’s surface is covered by water.
5.If 与whether均意为“是否”，但引导表语从句时，只能用whether, 不能用if。
The question is whether we can finish our work by tomorrow evening.
6.As, if, though引导的表语从句时，从句谓语多用虚拟语气。
这是因为从句中的情况与事实不相符。具体来说，如果从句表示与现在事实相反，谓语动词用一般过去式；
如果从句表示与过去事实相反时，谓语动词要用“had +过去分词”，如果从句表示将来的可能性不大，用would (might ,could )+动词原形，如：
Li Lei is now in a new jacket he looks as if he were an American boy.
但是，如果as if ,as though 引导的表语从句所表示的与事实相符，从句则用陈述语气，如：
The clouds are gathering. It looks as if is going to rain.
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