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。

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。