1 when引导的从句的谓语动词可以是延续性的动词,又可以是瞬间动词.并且when有时表示"就在那时".
例如：When she came in,I stopped eating.她进来时,我停止吃饭.(瞬间动词)
When I lived in the countryside,I used to carry some water for him.
当我住在农村时,我常常为他担水.（延续性的动词）
We were about to leave when he came in.我们就要离开,就在那时他进来了.
2 While引导的从句的谓语动词必须是延续性的,并强调主句和从句的动作同时发生（或者相对应）.并且while有时还可以表示对比.
例如：While my wife was reading the newspaper,I was watching TV.(was reading是延续性的动词,was reading和was watching同时发生)
I like playing football while you like playing basketball.我喜欢踢足球,而你喜欢打篮球.（对比）