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1．條件狀語從句：
1）Lets go out for a walkunlessyou are too tired.（除非，若不；相當於if---not）
即：If you are too tired,we’ll not go out for a walk.
2）You may borrow my bookas long asyou keep it clean.（只要，表示條件的唯一性）
3）Take your umbrellain caseit rains.（以防---，以免---）
4）I can tell you the truthon condition thatyou promise to keep a secret.（條件是---）
5）Supposingit rains, shall we continue the sports meeting?（如果，假如）
6）He wont be against us in the meetingprovided/providingthat we ask for his advice in advance.（假如，除非以……為條件）
7）You will be successful in the interviewonceyou have confidence.（一旦---就--）
1限定性定語從句She is the student who can speak English well.
2非限定性定語從句She is the student,who can speak English well.
一．主語從句
主語從句是在複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引導強調句的比較
It 作形式主語代替主語從句，主要是為了平衡句子結構，主語從句的連接詞沒有變化。而it引導的強調句則是對句子某一部分進行強調，無論強調的是什麼成分，都可用連詞that。被強調部分指人是也可用who/whom。例如：
It is a pity that you didn’t go to see the film.
It doesn’t interest me whether you succeed or not.
It is in the morning that the murder took place.
It is John that broke the window.
2、用it 作形式主語的結構
（1） It is ＋名詞＋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＋從句
It is natural that… 很自然…
It is strange that… 奇怪的是…
（3） it is ＋不及物動詞＋從句
It seems that… 似乎…
It happened that… 碰巧…
（4） it ＋過去分詞＋從句
It is reported that… 據報道…
It has been proved that… 已證實…
3、主語從句不可位於句首的五種情況
（1） if 引導的主語從句不可居於複合句句首。
（2） It is said ， (reported) …結構中的主語從句不可提前。例如：
It is said that President Jingo will visit our school next week. (right)
That President Jiang will visit our school next week is said. (wrong)
（3） It happens…， It occurs… 結構中的主語從句不可提前。例如：
It occurred to him that he failed in the examination. (right)
That he failed in the examination occurred to him. (wrong)
（4） It doesn’t matter how/whether …結構中的主語從句不可提前。例如：
It doesn’t matter whether he is wrong or not. (right)
Whether he is wrong or not doesn’t matter. (wrong)
（5） 含主語從句的複合句是疑問句時，主語從句不可提前。例如：
Is it likely that it will rain in the evening(right)
Is that will rain in the evening likely(wrong)
4、What 與that 在引導主語從句時的區別
What 引導主語從句時在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：
1) What you said yesterday is right.
2) That she is still alive is a consolation.
1主語從句Whether its right or not remains to be seen.
2賓語從句I wonder whether its right or not.
3同位語從句This is a question whether its right or not.
4表語從句The question is whether its right or not.
表語從句在複合句中作表語的名詞性從句，放在系動詞之後，一般結構是“主語＋連繫動詞＋表語從句”。可以接表語從句的連繫動詞有be, look, remain, seem等。引導表語從句的that常可省略。另外，常用的還有the reason is that… 和It is because 等結構。例如：
1) The question is whether we can make good preparation in such a short time.
2) This is why we can’t get the support of the people
3) But the fact remains that we are behind the other classes.
4) The reason he is late for school is that he missed the early bus.
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