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。 3. Were you writing when the teacher came in? 老师进来的时候，你在写信吗？ 4. Sorry, I was out when you called me. 对不起，你打电话来的时候我出去了。 5. He was on the point of leaving when someone knocked at the door. 他正要走，这时有人敲门。
6. I thought of it just when you opened your mouth. 就在你要说话的时候，我也想到了。 7. I had hardly[scarcely] closed my eyes when someone knocked at the door. 我刚一闭上眼，就有人在敲门了。
根据以上的例句，我们可以总结出一点：when 从句的A事件，相当于另一个事件B发生的时间点。也就是说，when 从句的重点不在动作本身发生的状态，而只是把它作为一个时间点，所以when 多数情况下用的是一般过去时，则不用正在进行时。因为如果用正在进行时，它表示的就是一段时间而不是一个时间点了。根据这一点，有的文章补充说：when 从句的动词大多是瞬时动词。
实际上，when 从句也可以有其它的时态，但几乎也不用进行时，因为它也只是作为一个时间参照点。
1. When I got to the airport, the guests had left. 当我赶到飞机场时，客人们已经离开了。 2. When he had finished his homework, he took a short rest. 当他完成作业后，他休息了一会儿。
3. Why do you want a new job when you have got such a good one already? 你已经找到如此好的工作，为何还想再找新的？
4. You shall borrow the book when I have finished reading it. 在我读完这本书后，你可以借阅。
5. When the manager comes here for a visit next week, I’ll talk with him about this. 下周，经理来这参观时，我会和他谈谈此事。
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。 1. While my wife was reading the newspaper, 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。
3. While they were talking, the bell rang. 正在他们谈话的时候，上课铃响了。 4. You can’t do your homework while you’re watching TV. 你不能一边看电视一边做家庭作业。
5. While John was sitting biting his nails, I was working out a plan to get us home.
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约翰坐在那里咬指甲时，我正在制定一个回家的计划。从时间的角度来看，while 表示的是一段时间，是一个过程。这是while 的侧重点。因此，如果含有“一段时间”的含义的时候，就可以用while。
6. Strike while the iron is hot. 趁热打铁。
这句话中，是说趁着铁是热的这段时间，赶紧打铁。如果换成 when 意思就变了，相当于说铁只热了一下，打一下，然后铁就冷了。这显然不符合文意。 再例：
—I'm going to the post office.
—While you're there, can you get me some stamps?
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. The students took notes as they listened. 学生们边听课边做笔记。
3. As we talked on, he got more and more excited. 我们继续往下谈的时候，他越来越兴奋。 4. The little girls sang as they went. 小姑娘们一边走，一边唱。 5. Just as he caught the ball, there was a tearing sound. 当他抓住球的时候，有一种撕裂的声音。 as 表达的事件，往往只是主句动作发生的背景或条件时，as 只是一个次要的时间说明，不像while 从句有强调 while 动作本身的意思。因此，as 常常翻译成“随着……”之意。 1. As the time went on，the weather got worse. 随着时间的推移，气候更加糟糕。 2. The atmosphere gets thinner and thinner as the height increases. 随着高度的增加，大气越来越稀薄。
3. 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。 1. The sad mother sat on the roadside, shouting as she was crying. 伤心的妈妈坐在路边，边哭边叫喊。
2. As we were going out, it began to snow. 正当我们出门时，雪开始下起来。 3. He came in as I was going to bed. 我正要上床睡觉，他进来了。
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 making a phone call, I was writing a letter.
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当她在打电话时，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。
五、比较while, when, as
1）as, when 引导短暂性动作的动词。
Just as / Just when / When I stopped my car, a man came up to me. 我刹车后，有一个人向我走来。
2）当从句的动作发生于主句动作之前，只能用when 引导这个从句，不可用as 或 while。 When you have finished your work, you may have a rest. 干完活后，你可以休息一下。
3）从句表示“随时间推移”连词能用as，不用when 或while。
As the day went on, the weather got worse. 日子一天天过去，天气越变越坏。
六、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。
（在肯定句中可用before代替。例如：Let's get in the wheat before the sun sets.） 否定句：She didn't arrive until 6 o'clock. 她直到6点才到。
Don't get off the bus until it has stopped. 公共汽车停稳后再下车。
I didn't manage to do it until you had explained how. 直到你教我后，我才会做。 区别：
1） until可用于句首，而till通常不用于句首。
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。 2）until when 疑问句中，until要放在句首。例如：
---Until when are you staying? ---Until next Monday. 注意：否定句可用另外两种句式表示。 3）Not until…在句首，主句用倒装。
Not until the early years of the 19th century did man know what heat is. 直到19 世纪初，人类才知道热为何物。
Not until I began to work did I realize how much time I had wasted. 直到工作，才认识到我已蹉跎了许多岁月。 4）It is not until… that….
It was not until I began to work that I realized how much time I had wasted. 5）表示“一…就…”的结构
hardly/scarcely…when/before, no sooner…than 和as soon as都可以表示“一… 就…”的 I had hardly / scarcely got home when it began to rain. 刚回家，就下起雨来了。 I had no sooner got home than it began to rain. As soon as I got home, it began to rain.
注意：如果hardly, scarcely 或no sooner置于句首，句子必须用倒装结构。
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Hardly / Scarcely had I got home when it began to rain. No sooner had I got home than it began to rain.
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练习
在下列各句的空格中填入适当的连词
1．_________ he comes tomorrow, I shall ask where he has been. 2．_________ he was speaking, everybody listened carefully. 3．I saw her just _________ she was getting off the train. 4．Have a good look at that man _________ you pass him. 5．It was already eight o'clock _________ we got there. 6. I was about to go out _________ a visitor came.
7．We'll go to the country at the beginning of June, ______ the summer harvest will start. 8. He learned to speak German _________ he was in Berlin. 9. Henry is in charge of the office ________ Mr. Smith is away. 10. I listen to the recorder _________ I have time.
11. He had learned Chinese _________ he came to China.
12. _________ the work was done, we sat down to sum up experience. 13. I haven't seen him _________ he moved to the other side of the town. 14. I waited ________ he came back.
15. It was not ________ he took off his eyeglasses that I recognized him. 16. She likes everything to be in place ________ she starts to work. 17. The thieves ran away _________ they caught sight of the police. 18. They decided to go back home _________ their money ran out. 19. We played outside till sunset, _________ it began to rain。
20. __________ I get to the airport, I will phone you to pick me up. 21. They were about to leave ______ it began to rain. 22. He always stay in bed ______ lunch time.
23. I like playing tennis _________ my younger sister prefers watching ball games. 24. _________ I understand your viewpoint, I don’t agree with you. 25. _______ she grew older, she became more responsible.
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Key
1. When 2. While / When 3. as 4. when/as 5. when 7. when 8. while 9. while 10. whenever 11. before 13. since 14. till/until 15. until 16. before 17. as soon as 19. when 20. As soon as 21.when 22. until 23. while 25. As
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6. when 12. After 18. before 24. While
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