主語從句：
在復(fù)合句中充當(dāng)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(dāng)一個主語。 主語從句的時(shí)態(tài)：不受主句的時(shí)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(shí)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(dāng)謂語較短時(shí)）。
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(jī)何時(shí)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(jī)何時(shí)起飛。
當(dāng)what引導(dǎo)的主語從句表示“…的東西”時(shí)，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(xiàng)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(dān)當(dāng)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(dāng)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(qiáng)調(diào)句的比較
�、僦髡Z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
�、诙鴱�(qiáng)調(diào)句則不同，它的結(jié)構(gòu)是：It+be+被強(qiáng)調(diào)部分+that+從句
強(qiáng)調(diào)主語：It is the times that produce their heroes.時(shí)勢造英雄
強(qiáng)調(diào)賓語：It is English that Prof.Lin teaches us.
強(qiáng)調(diào)狀語：It was in shanghai that I saw the film.
判斷是否是強(qiáng)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(dú)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(rèn)為缺成分的地方，如果放進(jìn)去是一句完整的句子了，那就說明是強(qiáng)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(shí)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(bào)道……
It has been proved that… 已證實(shí)……
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(shí)，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(shí)的區(qū)別
What 引導(dǎo)主語從句時(shí)在從句中充當(dāng)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(dá)時(shí)間的幾個句型：一般用when或者是what time：
What time will the train leave?
由于時(shí)刻都是固定的，也可以用一般現(xiàn)在時(shí)代替一般將來時(shí)：
What time does the train leave?
(2)時(shí)間的表達(dá)方式：8：19(nineteen past eight),7:57(three to eight)，在時(shí)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?yàn)檎Z氣太重：
You had better give up smoking .
(4)sb leave sth +地點(diǎn)
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(bǔ)充：從句的語序永遠(yuǎn)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(shí)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(bào)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(rèn)為他是最好的選手。
It is estimated that the vase is 2000 years old.
據(jù)估計(jì)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(rèn)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(dāng)“及物動詞 + 賓語”較短時(shí)，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？