例句：
He is a bus driver. ----- He was a bus driver.
am的`過去式是“was ”，過去分詞是“been ”,主語為"I"；
例句：
I am a student. ---- I was a student.
are的過去式是“were”，過去分詞是“been ”。are是復數(shù)，主語用you， we ，they。
例句：
They are basketball players. ---- They were basketball players.