虚拟语气是说话者用来表示假设，或难以实现的情况，而非客观存在的事实，所陈述的是一个条件，不一定是事实，甚至完全与事实相反。此外，如需表达主观愿望或某种强烈的感情时，也可用虚拟语气。常用的虚拟语气if从句有三种用法表现在：
（1）对现在情况的虚拟；
（2）对过去情况的虚拟；
（3）对将来情况的虚拟；
具体解释如下：
（1）对现在情况的虚拟
条件从句中谓语动词用一般过去式，主句用“would/should/might/could+动词原形”形式，表示对现在不可能实现情况的假想。
主句：主语＋would/should/might/could＋动词原形
从句：动词过去式;例：
If I were the president, I would put more money in education.
如果我是总统，我会投入更多钱到教育上。
If there were no water on the planet, we would be dead now.
如果地球上没有水，我们现在就是死人了。
If he studied at this school, he would know the environment around the school very much.
如果他在这所学校学习的话，他会很了解周边环境。
If I could do magic, I would kill the virus.
如果我会魔法，我将杀死病毒。
（2）对过去情况的虚拟
条件从句中谓语动词用过去完成式，主语用“would/could/should/might +have+动词的过去分词”形式。
提出过去的未发生的条件
主句：主语＋would/should/might/could have＋动词过去分词
从句：had＋动词过去分词
例：If I had got there earlier, I should have met her.
如果我当时早一点到哪儿，我就能见到她了。
If I had had 1000,000 dollars, I would have bought that car.
要是我那时有100万，我就买下那辆车了。
I would have passed the exam if I had studied harder.
如果我当初学习再努力一些，我就通过考试了。
If I had read the book, I would have told you about it.
如果我当时已经阅读了那本书，我当初就应该告诉你这个事情。
The problem could have been solved if the boss had put more fund into it.
如果当初老板投入更多资金的话，这个问题可能就解决了
The house might have been sold if the owner had failed to repay his debt.
如果当初房主没能还清贷款的话，这个房子可能就会被卖掉了。
If the task had been given more attention, it could have been finished. 如果当初这项任务多得到点关注，它有可能已经完成了。
I would have been more supportive if you had been more honest.
如果你当初更坦诚点，我会更支持你的。
（3）对将来情况的虚拟
表示与将来事实相反的条件从句中，谓语动词可用should do，were to do或动词的过去式三种形式表示，但意义略有不同。should表示的可能性最大，动词过去式的可能性次之，were to do 的可能性最小。
主句：主语＋would/should/might/could＋动词原形
从句：should＋动词原形/were to＋动词原形。
例：If he should go to Harvard University, he would make full use of his time.
如果他要上哈佛大学的话，他就会充分利用他的时间了。
If he did my homework, I would watch a film this evening.
If he were to do my homework, I would watch a film this evening.
If he should do my homework, I would watch a film this evening.
如果他今晚给我写作业，我就能看个电影了。
针对性训练：
1. —I still haven’t thanked Aunt Lucy for her present.
—It’s time you _____.
A. do
B. did
C. had
D. would
【答案与解析】
选B。It’s time you did 为 It’s time you thanked Aunt Lucy for her present 之略。按照英语语法，it’s time 后从句通常要用过去式。
2. If I hadn’t been lucky enough to meet you. I really _____ what I would have done.
A. don’t know
B. hadn’t known
C. wasn’t knowing
D. wouldn’t know
【答案与解析】
选A。虽然前有虚拟条件句, 后有使用了虚拟语气的宾语，但 I don’t know 却宜用一般现在时，因为 I don’t know 表述的是现在的真实情况，句意为“我当时要不是有幸遇到你，我真不知(指现在不知)我会做出什么蠢事来”。
3. —I’ve told everyone about it.
—Oh, I’d rather you _____.
A. don’t
B. hadn’t
C. couldn’t
D. wouldn’t
【答案与解析】
选B。I’d rather 后接从句时，从句谓语的时态规律是：用过去式表示现在或将来，用过去完成式表示过去。