when, while 和 as 引导时间状语从句的用法。
一、when 的用法。
如果只从现象来看，when 从句用的最多的是一般过去时，而主句的时态没有限制，根据具体情况而定。
1. when he was a child he was always trying out new ideas.
他小时候就常常试验一些新的设想。
2. when she came into my room, i was just reading a book. 她走进我房间时，我正在看书。
二、while 的用法。
相比于when 来说，while 从句的侧重点就不一样了。while 从句的侧重点在于描述动作正在发生的状态，它的意思是：当while 事件正在发生的时候，另一件事如何如何。
所以，while 从句一般用的是正在进行时。而另一件事的状态没有硬性的要求，根据具体情况而定。例如：
1. while my wife was reading the news*****, i was watching tv.
当妻子正在看报纸的时候，我正在看电视。
2. while jim was mending his bike, lin tao came to see him.
正当吉姆修自行车时，林涛来看他。
三、as 的用法。
as 从句表示的也是一件事情正在发生，另一件事也正在进行当中。但与 while 从句不同的是，as 从句用的一般不用正在进行时，而只是一般过去时。as 从句一般可以翻译成“边……边……”例如：
1. as my mother sang those old songs, tears ran down her cheeks.
当我妈妈唱起那些老歌时，眼泪顺着她的脸颊流了下来。
2. as years go by，china is getting stronger and richer.
随着时间一年一年过去，中国变得越来越富强了。
少数情况下，如果强调动作正在发生，as 从句也可以用正在进行时。这只能算是特例了。
2. as we were going out, it began to snow. 正当我们出门时，雪开始下起来。
四、when, while, as 的互换。
如果从句动作和主句动作同时发生，并且从句动作为延续性动词时，when，while，as 可以互换使用。这种情况下，它们的细微区别恐怕连英、美人自己也说不清了。
1. when /while /as we were dancing, a stranger came in.
当我们跳舞时，一位陌生人走了进来。 [dance 为延续性动词]
2. when /while /as she was ****** a phone call, i was writing a letter.
当她在打**时，我正在写信。 [make为延续性动词]
3. while/when/ as i was walking down the street, i noticed a police car in front of the bank.
我顺着马路往前走时，发现银行门前停着一辆警车。
2）比较until和till
此两个连词意义相同。肯定形式表示的意思是“做某事直至某时”，动词必须是延续性的。否定形式表达的意思是“直至某时才做某事”。
动词为延续性或非延续性都可以。正确使用这两个连词的关键之一就在于判断句中的动词该用肯定式还是否定式。
肯定句：i slept until midnight. 我一直睡到半夜时醒了。
wait till i call you. 等着我叫你。
否定句：she didn't arrive until 6 o'clock. 她直到6点才到。
区别：1）until可用于句首，而till通常不用于句首。
例如：until you told me, i had heard nothing of what happened.
直到你告诉我以前，出了什么事我一点也不知道。
六、练习：在下列各句的空格中填入适当的连词。
1he comes tomorrow, i shall ask where he has been.
2he was speaking, everybody listened carefully.
3．i saw her justshe was getting off the train.
4．h**e a good look at that manyou pass him.
5．it was already eight o'clockwe got there.
6. i was about to go outa visitor came.
8. he learned to speak germanhe was in berlin.
9. henry is in charge of the officemr. smith is away.
10. i listen to the recorderi h**e time.
11. he had learned chinesehe came to china.
12the work was done, we sat down to sum up experience.
13. i h**en't seen himhe moved to the other side of the town.
14. i waitedhe came back.
15. it was nothe took off his eyeglasses that i recognized him.
16. she likes everything to be in placeshe starts to work.
17. the thieves ran awaythey caught sight of the police.
18. they decided to go back hometheir money ran out.
19. we played outside till sunsetit began to rain。
20i get to the airport, i will phone you to pick me up.
21. they were about to le**e __it began to rain.
22. he always stay in bed __lunch time.
23. i like playing tennismy younger sister prefers watching ball games.
24i understand your viewpoint, i don’t agree with you.
25she grew older, she became more responsible.
26he was speaking, everybody listened carefully
was already 8:00 o’clockwe got there
28the work was done, we sat down to sum up experience.
waitedhe came back
key: 1. when 2. while / when 3. as 4. when/as 5. when 6. when
8. while 9. while 10. whenever 11. before 12. after
13. since 14. till/until 15. until 16. before 17. as soon as 18. before
19. when 20. as soon as 22. until 23. while 24. while
25. as
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