1. 与现在事实相反
条件从句用过去时（be动词的过去式一律用were），主句用would （should/ might / could）+动词原形”。
如： If I were you, I would accept the invitation to the party.
如果我是你的话，我将接受去舞会的邀请。
If I knew her well, I would tell you about her.
如果我很了解她的话，我将告诉你关于她的事情。
2. 与过去事实相反
条件从句用过去完成时，主句用“would /should/ might / could + have +过去分词”,
如： If you had been there last night, nothing would have happened.
如果你昨天晚上在那儿的话，就没有什么事情发生了。
If I had got up earlier, I would have caught the first bus.
如果我起得更早一些的话，就赶上头班车了。
3．与将来事实相反
条件从句用过去时或者“were to + 动词原形”或者“should+ 动词原形”，
主句用“would （should/ might / could）+动词原形”,
如： If it were Saturday tomorrow, we wouldn’t have an exam.
如果明天是周六的话，我们就不考试了。
If you were to visit the school tomorrow, you would see Professor Smith.
如果你明天去参观学校的话，你就会看到史密斯教授。
If it should rain tomorrow, the sports meet would be put off until next Friday.
如果明天下雨的话，运动会将被推迟到下周五举行。
如果if虚拟条件句中有were, had, 或者should等词时，可以省略if，
把这些词放在主语前，形成倒装句。如：
If I should meet her (Should I meet her), I would tell her.
如果我遇见她的话，我就告诉她。
If I were in your position (Were I in your position), I would do it better.
如果我在你的位置上，我会做得更好。
If they had made preparations (Had they made preparations), they would have succeeded.
如果他们做了准备的话，他们就已经成功了。
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