在復(fù)合句中充當主語成分的句子叫做主語從句。常規(guī)主語從句，即句子在復(fù)合句中充當一個主語。 主語從句的時態(tài)：不受主句的時態(tài)影響和限制。
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) 引導(dǎo)主語從句連詞有that,whether,who,what,whatever等
(2) 連詞位于句首不能省略
(3) 主語從句大多數(shù)情況下視為第三人稱單數(shù)
主語從句引導(dǎo)詞：
1、由連詞that引導(dǎo)的主語從句。
例如：That you will win the medal seems unlikely.
你想獲得獎牌看起來是不可能的。
That you are so indifferent bothers me.
你如此冷淡使我很煩惱。
That she survived the accident is a miracle.
她在事故中幸免于難簡直是奇跡。
2、用連接代詞或連接副詞if, whether 引導(dǎo)的主語從句。
例如： Which of them escaped from the prison is still a mystery.
他們哪一個是從監(jiān)獄里逃跑的，仍然是個謎。
When they will come hasn’t been made pubic.
他們什么時候來還不知道。
Whether she is coming or not doesn’t matter too much.
她來不來都無關(guān)緊要。
3、用關(guān)系代詞引導(dǎo)的主語從句。
例如：What you need is more practice.
你所需要的是更多的訓(xùn)練。
What I want to know is this. 我想知道的就是這事。
Whatever we do is to serve the people.
我們無論做什么都是為人民服務(wù)。
4、主語從句可以直接放在主語位置上，也可以用it作形式主語，而將從句放在句末（尤其是當謂語較短時）。
例如：That light travels in straight lines is known to all.
光沿直線傳播，這是眾所周知的事。
=It is known to all that light travels in straight lines.
眾所周知光沿直線傳播。
When the plane is to take off has not been announced .
飛機何時起飛還沒有宣布。
= It has not been announced when the plane is to take off.
還沒有宣布飛機何時起飛。
當what引導(dǎo)的主語從句表示“…的東西”時，一般不用it作形式主語。
錯:It is a book what he wants.
對:What he wants is a book. 他想要的是本書。
如果帶主語從句的句子是疑問式，則必須用it作形式主語的結(jié)構(gòu)。
例如：Has it been settled who will be sent to carry out the task ?
誰將被派去執(zhí)行這項任務(wù)決定了嗎 ？
主語從句與賓語從句區(qū)別：
主語從句與賓語從句兩者的用法和在句中的作用是不同的，下面從它們的各自用法來介紹。
一．主語從句
主語從句 定義：在句子中擔當主語的是一個從句，這個從句就叫做主語從句。
主語從句是在復(fù)合句中充當主語的從句，通常放在主句謂語動詞之前或由形式主語it代替，而本身放在句子末尾。
1. It 作形式主語和it引導(dǎo)強調(diào)句的比較
①主語從句常用it作形式主語，一般常用句型為：It is+{名詞/形容詞/過去分詞}+主語從句例如：
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，還有一些比較多見的結(jié)構(gòu)：
It turned out that……;
It has been proved that……;
It happened/occurred that……;
It is well-known that……等等
②而強調(diào)句則不同，它的結(jié)構(gòu)是：It+be+被強調(diào)部分+that+從句
強調(diào)主語：It is the times that produce their heroes.時勢造英雄
強調(diào)賓語：It is English that Prof.Lin teaches us.
強調(diào)狀語：It was in shanghai that I saw the film.
判斷是否是強調(diào)句有一個方法，就是將that以后的“句子的其余部分”拿出來單獨看，看有沒有缺成分，缺的是什么成分，再把謂語動詞后面that前面的那部分，帶到你認為缺成分的地方，如果放進去是一句完整的句子了，那就說明是強調(diào)句。
2. 用it 作形式主語的結(jié)構(gòu)
(1) It is +名詞+that從句
It is a fact that … 事實是……
It is an honor that …非常榮幸
It is common knowledge that …是常識
(2) it is +形容詞+that從句
It is natural that… 很自然……
It is strange that… 奇怪的是……
(3) it +不及物動詞+that從句
It seems that… 似乎……
It happened that… 碰巧……
(4) it is+過去分詞+that從句
It is reported that… 據(jù)報道……
It has been proved that… 已證實……
3.主語從句不可位于句首的五種情況
(1) if 引導(dǎo)的主語從句不可居于復(fù)合句句首。
(2) It is said , (reported) …結(jié)構(gòu)中的主語從句不可提前。例如：
It is said that President Jingo will visit our school next week.
(3) It happens…, It occurs… 結(jié)構(gòu)中的主語從句不可提前。例如：
It occurred to him that he failed in the examination.
(4) It doesn’t matter how/whether …結(jié)構(gòu)中的主語從句不可提前。例如：
It doesn’t matter whether he is wrong or not.
(5) 含主語從句的復(fù)合句是疑問句時，主語從句不可提前。例如：
Is it likely that it will rain in the evening?
4. What 與that 在引導(dǎo)主語從句時的區(qū)別
What 引導(dǎo)主語從句時在從句中充當句子成分，如主語．賓語．表語，而that 則不然。例如：
What you said yesterday is right.
賓語從句用以區(qū)分主語從句的幾個特征：
1、引導(dǎo)詞：what\which\whose\when\whether\if\where
2、語序：賓語從句必須是用陳述語句。
如：I think that you must work harder.
賓語從句的引導(dǎo)詞、連接詞的區(qū)別、否定轉(zhuǎn)移等現(xiàn)象。
(1)表達時間的幾個句型：一般用when或者是what time：
What time will the train leave?
由于時刻都是固定的，也可以用一般現(xiàn)在時代替一般將來時：
What time does the train leave?
(2)時間的表達方式：8：19(nineteen past eight),7:57(three to eight)，在時間的中間也可以加上分的單詞(minutes)
(3)had better +動詞原型。意思是提要求，建議。但是上下級不能用這種方式，因為語氣太重：
You had better give up smoking .
(4)sb leave sth +地點
I left my book in my classroom yesterday.
(5)may I have your name(address\age)?一般詢問對方的年齡、名字等，不要習(xí)慣以前的問法，這樣很不禮貌，而是要采訪上面的問法。
以前的習(xí)慣是：how old are you? \what's your name?
(6)修飾名詞的代詞次序：限-描-大-形---年---顏----籍----物--------類+名詞：
this is a bridge.
This is a beautiful bridge
補充：從句的語序永遠是陳述句。
主語從句結(jié)構(gòu)：
1、It is +名詞+從句
It is a fact that… 事實是……
It is good news that … ……是好消息
It is a question that … ……是個問題
It is common knowledge that … ……是常識
類似的名詞還有：a pity；a wonder；a good thing； no wonder； surprise等。
例如：It is a mystery to me how it all happened.
這一切是怎么發(fā)生的是個謎。
It is common knowledge that the whale is not a fish.
鯨魚不是魚，這是常識。
It is no surprise that Bob should have won the game.
鮑勃會贏得這場比賽，這不足為奇。
2、It is +形容詞+從句
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……
類似的形容詞還有：
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.
很明顯導(dǎo)體和絕緣體在工業(yè)中都很重要。
It is doubtful whether she will be able to come.
她是否能來令人懷疑。
It was really astonishing that he refused to talk to you .
他拒絕和你說話真是令人驚訝。
It is essential that he should be here by the weekend .
周末之前，他應(yīng)該到這里是必要的。
It seems obvious that we can not go on like this.
很明顯我們不能這樣下去了。
3、It is +過去分詞+從句
It is said that … 據(jù)說……
It is reported that … 據(jù)報道……
It has been proved that … 已證明……
It must be proved that… 必須指出……
類似的過去分詞還有：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.
大家都認為他是最好的選手。
It is estimated that the vase is 2000 years old.
據(jù)估計這個花瓶有2000年的歷史。
It is used to be thought that a new star must be due to a collision between two stars.
過去一直認為新星是由于兩顆星星之間碰撞產(chǎn)生的。
It has not been made clear when the new road is to be opened to traffic.
還沒弄清楚這條路將開始通車。
4、It +不及物動詞+從句
It seems that … 好像是……
It happened that… 碰巧……
It follows that … 由此可見……
It has turned out that … 結(jié)果是……
類似的不及物動詞還有：seem, appear, occur, follow, happen, turn out, etc.
例如：It turned out that nobody remembered the address.
結(jié)果是無人記得那個地址。
It now appears that they are in urgent need of help.
看起來他們急需幫助。
It does not matter if I missed my train, because there is another later.
沒有趕上這趟火車沒有關(guān)系，稍晚還有一趟。
It happened that I saw him yesterday.
碰巧我昨天看見他了。
當“及物動詞 + 賓語”較短時，也可用這種結(jié)構(gòu)。
例如：It shocked me that Peter did not tell anybody where he was.
讓我吃驚的是彼得沒有告訴任何人他在哪里。
It does not interest me whether you go or not.
我對你去不去不感興趣。
It +某些固定結(jié)構(gòu)+ 主語從句。
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 .
下不下雨都沒有多大關(guān)系。
It does not make any difference whether it rains or not .
下不下雨沒什么分別。
It does not make the least difference to me what you do.
對我來說你做什么都無關(guān)緊要。
Is it of much consequence to you that Mr. Wang has failed to do it ? 王先生沒有做成這件事對你有什么重要影響嗎？
相關(guān)初中英語知識點：表語從句
表語從句：
就是用一個句子作為表語。說明主語是什么或者怎么樣，由名詞、形容詞或相當于名詞或形容詞的詞或短語充當，和連系動詞一起構(gòu)成謂語。
表語從句是名詞性從句的一種。
表語從句的基本結(jié)構(gòu)是：
“主語+連系動詞+關(guān)聯(lián)詞+表語從句”。
連接表語從句的連接詞有：
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語序：
從句的語序必須是陳述句語序，即“主語+謂語”這種形式。
②時態(tài)：
當主句是一般現(xiàn)在時或一般將來時的時候，從句可以是任何時態(tài)，而當主句是一般過去時的時候，從句，從句時態(tài)必須是過去時范圍的時態(tài)，即（一般過去時，過去進行時，過去完成時，過去將來時）。
③連接詞：
當從句意思完整，主句意思肯定時，連接詞用that，且可以省去，當從句意思完整，主句意思不確定或含否定含意時，常用if或whether（是否），當從句意思不完整時，連接詞則是代替不完整部分的特殊疑問詞。
表語從句種類：
根據(jù)關(guān)聯(lián)詞來區(qū)分,表語從句可以分為六種:
一、由that引導(dǎo)的表語從句
The trouble is that he misunderstood me.麻煩的是他誤解了我。
有時，引導(dǎo)表語從句的連詞that可以省略
The truth is (that) I didn’t finish my term essay. 事實是我沒有寫完學(xué)期論文。
二、由連接代(副)詞引導(dǎo)的表語從句
連接代詞who, whom, whose, what, which, whoever, whatever, whichever；
連接副詞where, when, how, why。
That was what she said this morning in her bedroom.那就是她今天早晨在臥室中說的。
三、由關(guān)系代詞型的what引導(dǎo)的從句
I want to be a teacher. That’s what I want to be.
我想當老師，那是我的志愿。
四、關(guān)聯(lián)詞是連詞because
比較下面這兩個句子:
I was angry. That was because he didn’t understand me.
我生氣是因為他不理解我。(表語從句強調(diào)原因)
That’s why he got fired from that firm.那正是他被公司解雇的原因。(表語從句強調(diào)結(jié)果)
注意：reason作主語或主語中包含事件的起因時,后面的表語從句表示原因時要用that來引導(dǎo),而不用because
That is why ...是常用句型,其中why引導(dǎo)的從句在句中作表語,該句型通常用于針對前面已經(jīng)說過的原因進行總結(jié)。
1. That is the reason why ...That is the reason why ...與That is why ...是同義的，
但是從語法結(jié)構(gòu)上講，That is the reason why ...中的why引導(dǎo)的是一個定語從句，將其中的the reason去掉后則與That is why ...結(jié)構(gòu)一樣了
2. That is because ...That is because ...也是一個常用句型。
That is because ...句型中，連詞because引導(dǎo)的從句作表語，
That is because ...與That is why ...之間的不同在于That is because ...指原因或理由，
That is why ...則指由于各種原因造成的結(jié)果
He did not read Harry Potter last night. That is because he had to help his mother to do the housework.
昨天晚上他沒有讀《哈里波特》，那是因為他得幫助他媽媽做家務(wù)。
五、關(guān)聯(lián)詞是從屬連詞whether, as, as if
The question remains whether we can win the majority of the people.
問題是我們能否贏得大多數(shù)民眾的支持。
六. 使用虛擬語氣的表語從句
在表示建議、勸告、命令含義的名詞后的表語從句，謂語動詞需用“should+動詞原形” 表示，should可省略。
常見的詞有：advice，suggestion，order，proposal，plan，idea等。
My suggestion is that we （should） start early tomorrow.
我的建議是我們明天一早就出發(fā)。
表語從句中需要注意的地方:
1. 表語從句一定要用陳述語序；
2. 不可以用if,而用whether 連接表語從句(as if 例外)；
3. 與賓語從句不同,在有表語從句的復(fù)合句中,主句時態(tài)和從句時態(tài)可以不一致。
4. 連接詞that一般不能省略，但當主句中含動詞do的某種形式時，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邊去感謝他。
5. 當句子的主語是：advice, demand, decision, desire, idea, insistence, instruction, order, plan, proposal, recommendation, request, requirement, suggestion, wish等名詞時，
表語從句須用虛擬語氣，即：should +do。如：
My advice is that you should go to apologize to him. 我的建議是你應(yīng)該去向他道歉。
表語從句語法提示：
應(yīng)注意的問題：
1. 連系動詞be，appear，seem，look等之后可以跟表語從句。
It was because he didn't pass the exam.那是因為他沒有通過考試。
2. 引導(dǎo)表語從句的引導(dǎo)詞有以下幾類。
1）wh-疑問詞
My question is who left.我想問的是誰離開了。
2）whether
My question is whether he left（or not）。 我的問題是他是否離開了。
注：if不能引導(dǎo)表語從句。
3）that
The fact is that he left. 事實是他離開了。
注：引導(dǎo)表語從句的連詞that間或可以省略。
The truth is（that）I didn't go there. 事實是我沒去那兒。
4）because，as，as if，as though
It's just because he doesn't know her. 這是僅僅因為他不認識她。
3.表語從句要用陳述語序
That is where the famous scientist was born.
4.that和what在引導(dǎo)表語從句時，作用和意義都不相同。
That本身沒有詞義，在句中只起連接作用，不充當句子成分；what則表示“所……的（人或事）”，在表語從句中充當主語或賓語等。
That fact is that more than seventy percent of the earth’s surface is covered by water.
5.If 與whether均意為“是否”，但引導(dǎo)表語從句時，只能用whether, 不能用if。
The question is whether we can finish our work by tomorrow evening.
6.As, if, though引導(dǎo)的表語從句時，從句謂語多用虛擬語氣。
這是因為從句中的情況與事實不相符。具體來說，如果從句表示與現(xiàn)在事實相反，謂語動詞用一般過去式；
如果從句表示與過去事實相反時，謂語動詞要用“had +過去分詞”，如果從句表示將來的可能性不大，用would (might ,could )+動詞原形，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 引導(dǎo)的表語從句所表示的與事實相符，從句則用陳述語氣，如：
The clouds are gathering. It looks as if is going to rain.
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