1. when ,while ,as 引导的时间状语从句
1) when 引导的时间状语从句，意为“当……的时候”， when引导的从句中谓语可以用延续性动词，也可以用非延续性动词。
He was working at the table when I went in.
Things were different when I was young.
I was about to leave when the telephone rang.
2) while 引导的时间状语从句，表示“与……同时，在……期间”， while引导的从句中谓语常用延续性动词或表示状态的词。
