am的过去式为什么会变were

am的过去式是was，在过去时态里面我们通常是把am改成was，如：I was born in Shanghai.

但是在虚拟语气中am和is的过去式就会变成were。在if引导的虚拟语气中要对现在情况进行虚拟，则主语使用should/would/could+do，从句使用过去，be动词只能用were。如：I wouldn't go there if I were you.