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when和as 既可以和延续性动词连用，又可以和非延续性动词连用，而while只能和延续性动词连用。
一般常见的延续性动词有drink, eat, fly, have, keep, know, lie, live, keep, rain, read, run, sing, sleep, smoke, snow, stand, talk, wait, walk, wear, work, write, sit等
非延续性动词常见的有：arrive, begin, borrow, buy, break, close, come, die, fall, go, hit, jump, open, marry, lose, leave, put, start, stop, join等
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when 从句的谓语动词可以在主句谓语动词之前，之后或者同时发生， while, as从句的谓语动词必须和主句的谓语动词同时发生。
例如: When I had finished my homework, I took a rest.(finished先发生，这种情况只能用when)
当从句的动作和主句的动作同时发生，而且从句的动作为延续性动词时， 三者都可以用
例如：When/While/As my mother was watching TV, my father was writing a letter.
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当主句和从句的动作同时进行，且表示的意义为“随着…或一边…一边”时， 我们只能用as
例如:As the time went on, the weather got better.
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在将来时从句中，常用when, 且要遵循主将从现的原则
例如：When my teacher comes here, I will meet her.
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总结：(1)如果句子含有“随着”，“一边一边”的含义我们常用 as
(2)若暗示一种规律，表示“每当_的时候”要用when, 若主语与从句发生有先后顺序，我们要有when, when也可以做并列连词，表示“突然”的含义。
(3) 句中表示“当__时”，具有趁机的意味，通常用while, while也可以做并列连词，表示“而”，“却”的含义。
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习题巩固
1) ___ you meet a word you don`t know, consult the dictionary,
2)The students took notes____ they listened.
3) ___ the wind rose the noise increased.
4) She called____ I was out.
5) We must strike _____ the iron is hot.
6) ____ you have finished your work, you may have a rest.
7) I was just reading a book____ she came into my room.
答案： 1. When 2.as 3. As 4. while 5. while 6. When 7. when
