when, while和as的区别
when引导的从句的谓语动词可以是延续性的动词，可以是瞬时动词。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。(瞬时动词)
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。
While引导的从句的谓语动作必须是延续性的，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）
As表示“一边……一边”，as引导的动作是延续性的动作，一般用于主句和从句动作同时发生；as也可以强调“一先一后。例如：
We always sing as we walk.我们总是边走边唱。（as表示“一边……一边”）
As we were going out, it began to snow.当我们出门时，开始下雪了。（as强调句中两个动作紧接着先后发生，而不强调开始下雪的特定时间）
when引导的从句的谓语动词可以是延续性的动词，可以是瞬时动词。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。(瞬时动词)
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。
While引导的从句的谓语动作必须是延续性的，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）
As表示“一边……一边”，as引导的动作是延续性的动作，一般用于主句和从句动作同时发生；as也可以强调“一先一后。例如：
We always sing as we walk.我们总是边走边唱。（as表示“一边……一边”）
As we were going out, it began to snow.当我们出门时，开始下雪了。（as强调句中两个动作紧接着先后发生，而不强调开始下雪的特定时间）
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