1. 表示与现在事实相反的情况:

　　从句: If+主语+动词一般过去时 (be动词用were)

　　主句: 主语+ should/would/might/could+do

　　Eg. If I were you, I should take another measure to solve the problem.

　　翻译：如果我是你的话，我会采取另一种措施去解决这个问题。

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　　2. 表示与过去事实相反的情况:

　　从句: If+主语+had done

　　主句: 主语+should/would/might/could+have done

　　Eg. If we had known his address, we could have written to him.

　　翻译：如果我们知道他的地址的话，我们可以给他写信。

　　3. 表示与将来的事实相反的情况：

　　从句：If+主语+were to do/should do/动词过去时 (be动词用were)

　　主句: 主语+ should/would/might/could+do

　　Eg. If she were to arrive here tomorrow, we would give her a surprise.

　　翻译：如果她明天来这里，我们就给她一个惊喜。