If I were you 其中為什莫用were

虛擬語氣的時候動詞要往過去倒壹個時態.

現在變為過去

過去變為現在完成

現在完成變為過去完成.

If I were you的意思是假如我是妳.

這裏是把be動詞改為了過去式.

但是語法規定虛擬語氣裏面be動詞的過去式就是were

所以要說If I were you, If he were you

妳也可以倒裝說 Were I you， Were he you。^^