一．分别对过去、现在和未来的假设1、过去发生事情进行假设这时从句动词用had done，主句用would/should/could/might+have doneIf you had worked harder, you would have passed your exam.If you had asked me, I would have told you.I would have been in bad trouble if Jane hadn’t helped me.If it hadn’t been for your help, we could have been in lots of trouble.(要不是你的帮助，我们要陷入很大的麻烦)2、对现在情形进行假设这时从句用did和were（be 动词都改成were），主句为would should/could/might+doIf I knew her name, I would tell you.I would perfectly happy, if I had a car.If I were rich，I would spend all my time traveling.If my nose were a little shorter I’d be quite pretty.If it weren’t for the children, we could go skiing.要不是孩子们的原因，我们就会去滑雪3、对将来情形假设这时从句有以下三种动词形式 1.were/did 2.were to 3.should 主句为would/should/could/might+do看看下面的例子What would I do if I were to lose/should lose/lost my job？If you were to move your chair a bit, we could all sit down.if it should rain/were to rain/rained tomorrow, I would not go out.If they should play/were to play / played better next time , they would not fail again上述主句中would最为常用，might=would perhaps/would possibly表可能如If you asked me nicely, I might buy you a drink.could=would be able to表能力如If I had another $500, I could buy a car.二、If it weren’t for…和It hadn’t been for…If it weren’t for…和If it hadn’t been for…，其意为“若不是(有)” “要不是”。

如：If it weren’t for water, no plant could grow. 要是没有水植物就无法生长。

If it hadn’t been for your assistance, we wouldn’t have succeeded.=But for your assistance, we wouldn’t have succeeded.=Without your assistance, we wouldn’t have succeeded.三、含蓄条件句有些句子中虽然没有出现if，但因为but for （要不是），otherwise（否则）等词也暗含着假设，所以要用if假设句的主句动词形式。

如：He would have given you more help,but he was too busy他本来会给你更多的帮助，但是他太忙了。

也就是说，如果那时他不忙，他可以给你更多的帮助。

句中but he was too busy实际上暗示了一个虚拟条件——如果那时他不忙He would lose weight, but he eats too much他本来可以减肥的，但是他吃的太多了。

也就是说，如果他吃得不多的话，他是可以减肥的。

句中的but he eats too much实际上暗示了一个虚拟条件——如果他吃得不多。

Without cars human life would be quite different today. (If there weren’t cars, human life would be quite different today)I am glad you told me about the show being canceled. Otherwise I would havetraveled all the way to London for nothing.(Otherwise=If you hadn’t told me about the show being canceled)He telephoned to inform me of your birthday, or I would have known nothing about it.I might have given you more help, but I was too busy.But for Tom, we could have won the game. (But for Tom=If it hadn’t been for Tom)He telephoned to inform me of your birthday, or I would have known nothing about it.I might have given you more help, but I was too busy.What would you do with a million dollars? ( = if you had a million dollars)We couldn't have finished the work ahead of time without your help. ( = if we hadn't got your help)But for the rain ( = If it hadn't been for the rain ), we would have finished the work.▲四、混合语气前面我们讨论的都是主句和从句在同一时间发生的。

有时主句与从句的动作发生在不同的时间，主句和从句的动词形式因时间不同而不同，此时应根据具体的语境情况，结合上面提到的三种基本类型对时态作相应的调整：我们把它称为混合语气，比如If you had asked him yesterday, you would know what to do now.（从句是过去的假设，主句是现在的结果）If it had rained last night(过去)，it would be very cold today (现在)If I knew you were coming(现在), I’d have baked a cake（过去）.You would be much better now if you had taken my advice. 假若你当时听我的话，你现在就会好多了。

If you had studied hard before, you would be a college student now, and you would graduate from a college i n four years’ time.▲五、假设句的省略与倒装如果if从句中有were，should，had，可以把if省掉，把上述词提前即可：Were they here now, they could help us. = If they were here now, they could help us.他们现在在的话，就会帮助我们了。

Had you come earlier, you would have met him. = If you had come earlier，you would have met him.你来得早一点，就碰到他了。

Should it rain, the crops would be saved. = If it should rain, the crops would be saved.假如下雨，庄稼就有救了。

Had we not missed the plane, we would have been killed in the crash. = if we hadnt missed the plane, we would have been killed in the crash.(不能写成Hadn’t we missed.即had, were, should 提前不能连带not一起写成缩略形式) Were it not for water, no plant could grow.五、假设句的练习2._____ fired, your health care and other benefits will not be immediately cut off. （06湖北卷）A. Would you beB. Should you beC. Could you beD. Might you be答案为B。

本题句意为：如果你被炒了，你的医疗保险和其他利益并不会立即取消。

与将来事实相反。

3.—Is he feeling any better?—No. If he had followed the doctor’s advice, he _____ quite all right now.A. will have beenB. would have beenC. would beD. will be答案为C。

本题为混合时间的虚拟条件句,从句指过去情况，主句为现在情况。

句意：如果他那时接受了医生的建议，现在他就好了。

另外，if引导的虚拟条件句的从句中若含有有were,should或had等，可将if 省略，把were, should或had移至主语前。