as、while 和 when 引导时间状语从句时的区别：
一、意为“当......的时候”
when从句中既可以用延续性动词，表示状态或时间段，也可以用非延续性动词，表示动作或时间点。
It was snowing when we arrived at the station.
我们到达车站时正在下雪。
When he came in, we all stood up, smiling.
当他进来时，我们都站了起来，微笑着。
while 从句中只能用延续性动词，从句一般用的是进行时。而另一件事的状态没有硬性的要求，根据具体情况而定，表示状态或时间段。
While she was typing, someone knocked at the door.
她打字时，有人敲门。
You can go swimming while I'm having lunch.
我吃午饭时你可以去游泳。
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，从句一般不用进行时，而只是一般过去时，译成“边……边……”。例如：
He sat watching her as she got ready.
他一直坐着看她准备停当。
As she grew older she gained in confidence.
随着年龄的增长她的信心增强了。
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。
The students took notes as they listened.
学生们边听课边做笔记。
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。
The little girls sang as they went.
小姑娘们一边走，一边唱。
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。
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。
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。
As we were going out, it began to snow.
正当我们出门时，雪开始下起来。
He came in as I was going to bed.
我正要上床睡觉，他进来了。
二、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。
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。
解析：dance 为延续性动词。
When /While /As she was making a phone call, I was writing a letter.
当她在打电话时，我正在写信。
解析：make 为延续性动词。
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。
解析：walk 为延续性动词。
三、比较while, when, as
1、as, when 引导短暂性动作的动词。例如：
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。例如：
When you have finished your work, you may have a rest.
干完活后，你可以休息一下。
I lost my job when the company moved to another state.
公司搬到了另一个州，我便失业了。
3、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。
As the day went on, the weather got worse.
日子一天天过去，天气越变越坏。
As the time went on，the weather got worse.
随着时间的推移，气候更加糟糕。
The atmosphere gets thinner and thinner as the height increases.
随着高度的增加，大气越来越稀薄。
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。
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