对过去的虚拟if虚拟：从句 if + had done（过去完成） 主句 would/could/should/might + have done（现在完成）
对现在的虚拟：从句 if + did（过去式） 主句 would/could/should/might + do（动词原型）
对将来的虚拟：从句1) if + should + do（动词原型）; 2) if + were to +do（动词原型）; 3) if + did（过去式）; 主句 would/could/should/might + do（动词原型）
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，不是事实，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.
不是的
某事很可能发生,条件是可能存在的,主句中某种情况发生的概率也是很高的.【主将从现】，即真实条件句，如：
If you ask him,he will help you．如果你请他帮忙,他会帮你的.
If you fail in the exam,you will let him down．如果你考试不及格,你会让他失望的.
If you have finished the homework,you can go home．如果你作业做完了就可以回家了.
if从句还表示不可实现的条件或根本不可能存在的条件,也就是一种虚拟的条件或假设.从句多用一般过去时或过去完成时,表示对现在或过去的一种假设.即非真实条件句，【虚拟语气】如：
If I were you,I would invite him to the party．如果我是你,我会邀请他参加聚会.
I would have arrived much earlier if I had not been caught in the traffic．要不是交通堵塞,我本会来得早一些.