條件狀語從句，由if, as (so) long as和unless引導。
條件狀語從句分為兩大類：真實條件句和虛擬條件句。當從句所表達的條件是有可能實現的、有可能是真實的條件時，用真實條件句；如果從句所表達的條件是不可能實現的、與客觀事實完全相反的條件或假設時，用虛擬條件句。
真實條件句在使用中也有需要注意的事項：從句中的動詞應該用一般現在時代替一般將來時；用現在完成時代替將來完成時。
eg：You can arrive in Beijing earlier for the meeting provided you don't mind taking the night train.
一、混合虛擬條件句
如果條件從句的動作和主句的動作不是同時發生，主句和從句的謂語動詞的
形式應分別根據各自所表示的 時間加以調整。這種句子叫做混合虛擬條件句。
如：
If he had followed my advice,he would be quite all right now.（從句
述說過去，主句述說現在）
If China had not been liberated,the working people would still be
leading a miserable life.（ 從句表示與過去的事實相反，主句說明與現在的
事實相反）
二、省略if的倒裝條件句
如果條件從句的謂語動詞包含有were或助動詞、情態動詞
had,should,could，有時可將連詞if省去，而將 條件從句的主語置於
were,had,should,could之後。這種用法主要用於書面語，如：
Were I (=If I were) in your place,I wouldn't give it up so early.
如果我是你，我不會這么早就放棄。
Had I (=if I had) known,I might have joined you in the discussion.
如果（之前）我知道，我會參加到你們的討論中的。
Should you (=If you should) be interested,I have a book on the subject
you might like.
如果你有興趣，我有一本書，這本書有你或許喜歡的課題。
三、省略主句，只保留if(only)的條件句
虛擬條件句中的表示結果的主句有時形式上可以省略，但意義仍然存在。這
種句子只保留一個if(only)條件從句，表達說話人的強烈願望。如：If(only)she
were my sister！如果她是我的妹妹就好了！
四、含蓄虛擬條件句
有時虛擬條件句不用條件從句，而是通過上下文或用介詞短語等表示出來。
這種句子叫做含蓄虛擬條件句 。下面列舉幾種。
1.在主語詞組中表示含蓄條件
(1)主語是動詞不定式短語，表示含蓄條件。
It would be good to leave the matter in his charge.如果把這件事讓
他負責就好了。
該句可改寫成：It would be good if we could leave the matter in his
charge.
(2)主語是動名詞短語表示含蓄條件。
Doing the work in that way would have cost much more time and money.
如果用那種方法做這項工作就 會耗費更多的時間和金錢。
該句可改寫為：If we had done the work in that way,it would have cost
much more time and money .
(3)主語中帶有定語從句表示含蓄條件。
A man who is always thinking of others would not have done such a thing.
如果一個人總為別人著想 ，他就不會做出這種事情來。
(4)主語中含有作定語的分詞或形容詞表示含蓄條件。
Anyone being his position would have made no reply.任何人處在他那
個位置，都不會回答的。
該句可改寫為：If anyone had been in his position he would have made
no reply.
A less complicated problem would have easily been solved.如果問題
不是很複雜的話，那么就很容易 解決了。
該句可改寫為：If the problem had been less complicated,it would have
easily been solved.
2.用介詞短語表示含蓄條件
We could have done better under more favourable conditions.
如果在更好的條件下，我們會幹得更好。
此句可改寫為：If the conditions had been more favourable, we could
have done better.
This change could not have taken place without a solid industrial
foundation.
如果沒有堅定的工業基礎，發生這樣的變化是不可能的。
此句可改寫為：If it had been without a solid industrial
foundation,this change could not hav e taken place.
3.用but或but for引導含蓄條件句（but後跟從句，but for後跟短語）
But for your help,our experiment wouldn't have been sosuccessful.
假如沒有你們的幫助，我們的實驗是不會如此成功的。
But for the storm,we should have arrived earlier.
要不是碰到暴雨我們會到得早一些。
We would have invited them to the dance,but they were too busy.
要不是他們太忙，我們會邀請他們來參加舞會的。
此句可改寫為：If they had not been so busy,we would haveinvited them
to the dance.
But that he is in hospital,He would go abroad for his summer vacation.
要不是生病住院，他就出國度暑假了。
該句可改寫為：If he weren't in hospital,he would go abroad for his
summer vacatiom.
