条件状语从句
连接词主要有 if, unless, as/so long as, on condition that 等.if 引导的条件句有真实条件句和非真实条件句两种.非真实条件句已在虚拟语气中阐述.
unless = if not. 例如：
Let's go out for a walk unless you are too tired. 如果不太累,我们去散散步.
If you are not too tied, let's go out for a walk.
You will be late ___ you leave immediately.
A. unless B. until C. if D. or
答案A.句意：除非你立即走,否则你就回迟到的.可转化为 If you dong leave immediately, you will be late.B、D句意不对,or表转折,句子如为 You leave immediately or you will be late.
虚拟语气
1）虚拟语气用来表示说话人的主观愿望或假想,所说的是一个条件,不一定是事实,或与事实相反.虚拟语气在条件句中应用比较多.
2）条件句可分为两类,一类为真实条件句,一类为非真实条件句.非真实条件句表示的是假设的或实际可能性不大的情况,故采用虚拟语气.
真实条件句用于陈述语气,假设的情况有可能发生.
一般现在时
shall/will + 动词原形
祈使句 情态动词一般现在时
例如：If he comes, he will bring his violin. 如果他来,会带小提琴来的.
The volleyball match will be put off if it ___.
A. will rain B. rains C. rained D. is rained
答案B.真实条件句主句为将来时,从句用一般现在时.
注意：
1）在真实条件句中,主句不能用be going to表示将来,该用shall, will.
（错） If you leave now, you are never going to regret it.
（对） If you leave now, you will never regret it.
2）表示真理时,主句谓语动词不用shall （will） +动词原形,而直接用一般现在时的动词形式.
非真实条件句
1）虚拟语气可以表示过去,现在和将来的情况,时态的基本特点是时态往后推移.
a. 与现在事实相反的假设
一般过去时（be用were）should（would）等 +动词原形
例如：If they were here, they would help you. 如果他们在这儿,会帮助你的.
含义：They are not here, they can’t help you.
b. 与过去事实相反的假设
过去完成时
should（would）等+ have+ 过去分词
例如：If he had come yesterday, I should / would have told him about it.如果他昨天来的话,我会把这件事告诉他的.
含义：He did not come yesterday, so I did not tell him about it.
c. 表示对将来不大可能发生的事情的假想
一般过去时
should/would等 + 动词原形
were+ 不定式
should+ 动词原形
例如：If you succeeded, everything would be all right. 如果你将来成功了,一切都会好的.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
含义：You are not likely to succeed, everything will be what it is now.