例句：
He is a bus driver. ----- He was a bus driver.
am的`过去式是“was ”，过去分词是“been ”,主语为"I"；
I am a student. ---- I was a student.
are的过去式是“were”，过去分词是“been ”。are是复数，主语用you， we ，they。
They are basketball players. ---- They were basketball players.
