PETS

if引导的状语从句很多。它的一般形式是 主句将来时，从句一般现在时。

例如：If we finish the work today, we will have a party.

If you work for me, i will give you some money in return.

虚拟语气的典型体现就是在if.条件句中。

If i were young, I would take that job.

If you had been there, you would have helped me.

If you went there tomorrow, I would join you.

细节的内容你可以查一下关于虚拟语气的语法

相关知识：

if 的条件从句分为真实条件从句和虚拟条件从句。而真实条件从句适用于主将从现，是假设有可能发生的。而虚拟条件从句是假设不太可能发生的。

IF的虚拟语气有三种：

内容                      从句                  主句

与现在事实相反    动词过去式(be用were)    would/could/should/might+动原

与过去事实相反    had+动词过去分词      would/could/should/might+have+动分

与将来事实相反   should/were to+动原    would/could/should/might+动原

下面三句依次是上面三种情况的例句：

If i were you ,I would/could/should/might do that.

If i had know that yesterday,I would/could/should/might have told you .

If it should rain ,the crops would/could/might be saved.

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