条件状语从句分为两大类：真实条件句和虚拟条件句。当从句所表达的条件是有可能实现的、有可能是真实的条件时，用真实条件句；如果从句所表达的条件是不可能实现的、与客观事实完全相反的条件或假设时，用虚拟条件句。
条件状语从句，由if, as (so) long as和unless引导。
条件状语从句分为两大类：真实条件句和虚拟条件句。当从句所表达的条件是有可能实现的、有可能是真实的条件时，用真实条件句；如果从句所表达的条件是不可能实现的、与客观事实完全相反的条件或假设时，用虚拟条件句。
真实条件句在使用中也有需要注意的事项：从句中的动词应该用一般现在时代替一般将来时；用现在完成时代替将来完成时。
eg：You can arrive in Beijing earlier for the meeting provided you don't mind taking the night train.
一、混合虚拟条件句
如果条件从句的动作和主句的动作不是同时发生，主句和从句的谓语动词的
形式应分别根据各自所表示的 时间加以调整。这种句子叫做混合虚拟条件句。
如：
If he had followed my advice,he would be quite all right now.（从句
述说过去，主句述说现在）
If China had not been liberated,the working people would still be
leading a miserable life.（ 从句表示与过去的事实相反，主句说明与现在的
事实相反）
二、省略if的倒装条件句
had,should,could，有时可将连词if省去，而将 条件从句的主语置于
were,had,should,could之后。这种用法主要用于书面语，如：
Were I (=If I were) in your place,I wouldn't give it up so early.
如果我是你，我不会这么早就放弃。
Had I (=if I had) known,I might have joined you in the discussion.
如果（之前）我知道，我会参加到你们的讨论中的。
Should you (=If you should) be interested,I have a book on the subject
you might like.
如果你有兴趣，我有一本书，这本书有你或许喜欢的课题。
三、省略主句，只保留if(only)的条件句
虚拟条件句中的表示结果的主句有时形式上可以省略，但意义仍然存在。这
种句子只保留一个if(only)条件从句，表达说话人的强烈愿望。如：If(only)she
were my sister！如果她是我的妹妹就好了！
四、含蓄虚拟条件句
有时虚拟条件句不用条件从句，而是通过上下文或用介词短语等表示出来。
这种句子叫做含蓄虚拟条件句 。下面列举几种。
1.在主语词组中表示含蓄条件
(1)主语是动词不定式短语，表示含蓄条件。
It would be good to leave the matter in his charge.如果把这件事让
他负责就好了。
该句可改写成：It would be good if we could leave the matter in his
charge.
(2)主语是动名词短语表示含蓄条件。
Doing the work in that way would have cost much more time and money.
如果用那种方法做这项工作就 会耗费更多的时间和金钱。
该句可改写为：If we had done the work in that way,it would have cost
much more time and money .
(3)主语中带有定语从句表示含蓄条件。
A man who is always thinking of others would not have done such a thing.
如果一个人总为别人着想 ，他就不会做出这种事情来。
Anyone being his position would have made no reply.任何人处在他那
个位置，都不会回答的。
该句可改写为：If anyone had been in his position he would have made
no reply.
A less complicated problem would have easily been solved.如果问题
不是很复杂的话，那么就很容易 解决了。
该句可改写为：If the problem had been less complicated,it would have
easily been solved.
2.用介词短语表示含蓄条件
We could have done better under more favourable conditions.
如果在更好的条件下，我们会干得更好。
此句可改写为：If the conditions had been more favourable, we could
have done better.
This change could not have taken place without a solid industrial
foundation.
如果没有坚定的工业基础，发生这样的变化是不可能的。
此句可改写为：If it had been without a solid industrial
foundation,this change could not hav e taken place.
3.用but或but for引导含蓄条件句（but后跟从句，but for后跟短语）
But for your help,our experiment wouldn't have been sosuccessful.
假如没有你们的帮助，我们的实验是不会如此成功的。
But for the storm,we should have arrived earlier.
要不是碰到暴雨我们会到得早一些。
We would have invited them to the dance,but they were too busy.
要不是他们太忙，我们会邀请他们来参加舞会的。
此句可改写为：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，他就出国度暑假了。
该句可改写为：If he weren't in hospital,he would go abroad for his
summer vacatiom.
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