am的過去式為什麼會變were

am的過去式是was，在過去時態裏面我們通常是把am改成was，如：I was born in Shanghai.

但是在虛擬語氣中am和is的過去式就會變成were。在if引導的虛擬語氣中要對現在情況進行虛擬，則主語使用should/would/could+do，從句使用過去，be動詞只能用were。如：I wouldn't go there if I were you.