If I were you 其中为什莫用were

这是虚拟语气.

虚拟语气的时候动词要往过去倒一个时态.

现在变为过去

过去变为现在完成

现在完成变为过去完成.

If I were you的意思是假如我是你.

这里是把be动词改为了过去式.

但是语法规定虚拟语气里面be动词的过去式就是were

所以要说If I were you, If he were you

你也可以倒装说 Were I you， Were he you。^^