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中考英语知识点：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，又可以是瞬时动词。并且when有时表示"就在那时"。

　　When she came in, I was eating.她进来时，我在吃饭。(瞬时动词)

　　When I lived in the countryside, I used to carry some water for him.当的住在农村时，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。

　　We always sing as we walk.我们总是边走边唱。(as表示"一边…一边")

　　As we was 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)

　　有时这三个连词可以互换，有时不可以。

　　lt was raining hard when (as) I got there.

　　我到那里时，正在下大雨。 ( 动作同时发生，when可换为as, 但不能换为while，因为get是点动词.)

　　When I had read the article, he called me.