ago置于時(shí)間段后,只用于一般過去時(shí),表示從現(xiàn)在算起的一段時(shí)間以前.
before置于時(shí)間段前,可用于一般過去式或過去完成時(shí).
before后接時(shí)間點(diǎn),可用于任何時(shí)態(tài)
before可單獨(dú)使用,置于句尾
例：He came back to his hometown 5 years ago.
They have ever been there before,
He always sleeps late , he never goes to bed before nine o'clock.