概述
一個句子在複合句中充當一個主語,那么這個句子就是主語從句.引導主語從句連詞that,whether,who,what,whatever等，連詞位於句首不能省略。引導詞
主語從句1、由連詞that引導的主語從句。
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. 她在事故中幸免於難簡直是奇蹟。
2、用連線代詞或連線副詞if, whether 引導的主語從句。
例如： Which of them escaped from the prison is still a mystery. 他們哪一個是從監獄裡逃跑的，仍然是個謎。
When they will come hasn’t been made pubic. 他們什麼時候來還不知道。
Whether she is coming or not doesn’t matter too much. 她來不來都無關緊要。
3、用關係代詞引導的主語從句。
例如：What you need is more practice. 你所需要的是更多的訓練。
What I want to know is this. 我想知道的就是這事。
Whatever we do is to serve the people. 我們無論做什麼都是為人民服務。
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作形式主語的結構。
例如：Has it been settled who will be sent to carry out the task ? 誰將被派去執行這項任務決定了嗎 ？
結構
1、It is +名詞+從句
It is a fact that… 事實是……
It is good news that … ……是好訊息
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. 很明顯導體和絕緣體在工業中都很重要。
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，他應該到這裡是必要的。
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 … 據說……
It is reported that … 據報導……
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於兩顆星星之間碰撞產生的。
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係，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，也可用這種結構。
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裡。
It does not interest me whether you go or not. 我對你去不去不感興趣。
It +某些固定結構+ 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係。
It does not make any difference whether it rains or not . 下不下雨沒什麼分別。
It does not make the least difference to me what you do. 對我來說你做什麼都無關緊要。
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麼重要影響嗎？
