1.定義：用作主語的從句叫做主語從句。
2.構(gòu)成：關(guān)聯(lián)詞+簡單句
3.引導主語從句的關(guān)聯(lián)詞有三類：
(1)從屬連詞that.
如：That they were in truth sisters was clear from the facial resemblance between them. 很明顯，她們確是親姐妹，她們的臉型很相似。
(2)從屬連詞whether。如：Whether he'll come here isn’t clear. 他是否會來這里還不清楚。
(3)連接代詞who, whom, whose, what, which, whoever, whatever, whichever
連接副詞where, when, how, why.如：
What she did is not yet known. 她干了什么尚不清楚。
How this happened is not clear to anyone. 這事怎樣發(fā)生的，誰也不清楚。
Whoever comes is welcome. 不論誰來都歡迎。
Wherever you are is my home-my only home. 你所在的任何地方就是我的家——我的家。
難點釋疑：
1.主語從句能用it作形式上的主語。常以it作形式主語的句型有：
A. It+be+形容詞(obvious, true, natural, surprising, good, wonderful, funny, possible, likely, certain, probable, etc.)+that從句。如：
It is certain that she will do well in her exam. 毫無疑問她考試成績會很好。
It is probable that he told her everything. 很可能他把一切都告訴她了。
B. It+be+名詞詞組(no wonder, an honour, a good thing, a pity, no surprise, etc.)+that從句。如：It's a pity that we can't go. 很遺憾我們不能去。
It's no surprise that our team should have won the game. 我們沒贏這場比賽真意外。
C. It+be+過去分詞(said, reported, thought, expected, decided, announced, arranged, etc.)+that從句。如：
It is said that Mr. Green has arrived in Beijing. 據(jù)說格林先生已經(jīng)到了北京。
It is reported that China has sent another man-made earth satellite into orbit. 據(jù)報道中國又成功地發(fā)射了一顆人造地球衛(wèi)星。
D. It+seem, happen等不及物動詞及短語+that從句。如：
It seems that Alice is not coming to the party at all. Alice似乎不來參加晚會。
It happened that I was out that day. 碰巧我那天外出了。
E. 當主語從句出現(xiàn)在感嘆句中時，要以it作形式主語，而把主語從句后置。如：
How strange it is that the children are so quiet! 孩子們這么安靜真奇怪!
2.注意連接代詞whoever, whatever, whichever等引導主語從句的含義：
Whoever comes will be welcome. (whoever=the person who) 來的人將受到歡迎。
Whatever he did was right. (whatever=the thing that) 他所做的事情是正確的。
Whichever of you comes in will receive a prize. (whichever=anyone of you who)
你們當中不論哪個進來將會得到獎勵。