as引导持续性动作，侧重主句和从句的动作同时发生。 He hurried home, looking behind as he went. I saw your sister as she was getting on the bus yesterday. He sang as he walked.

While：指的是“在某一时间里”，“在… 期间”，从句里的动作必须是持续性的，它也强调主句和从句动作的同时发生，往往侧重主句和从句动作的对比。

如： While he was in London, he studied music. While we were watching TV, he was writing a composition.