用法：when引导的时间状语从句中谓语动词可用延续性动词（表时间段）和终止性/结束性/短暂性动词(表时间点)

① 从句动作既可和主句的动作同时发生

② 又可在主句的动作之前或之后发生

2.while意思："当...的时候"，

用法：while引导的时间状语从句中谓语动词只能是延续性动词（时间段或状态）

① 强调主句的动作与从句的动作同时持续进行（主句中的谓语动词通常是延续性动词）

② 或者主句的动作发生在从句动作的进行过程中（主句中的谓语动词通常是终止性动词）

a. as意思："当...时"，"和...同时", "一边...一边..."，"随着..."

用法：as引导的动作是延续性的动作

① 侧重表示两个动作同时发生(包括一个主语同时进行两个动作)

② 或者一种动作随着另一种动作的变化而变化。

考题1］ I had just steeped out of the bathroom and was busily drying myself with a towel ____ I heard the steps. (2006湖南) A. while B. when C. since D. after

考题2］ He was about halfway through his meal ______________ a familiar voice came to his ears.

　　 A. why B. where C. when D. while

考题3］We were swimming in the lake _______ suddenly the storm started. (2004北京春)

　　 A. when　　　B. while C. until 　　D. before

考题4］- I'm going to the post office.

- ____ you're there, can you get me some stamps? (1999)

　　 A. As B. While C. Because D. If

考题5］_______________ the days went on, the weather got worse. (1990)

　　 A. With B. Since C. While D. As

考题6］I ____________ along the street looking for a place to park when the accident _______________.