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when, while, as区别详解
一、根据从句动作的持续性来区分
1. “主短从长”型：即主句是一个短暂性的动作，而从句是一个持续性动作，此时三者都可用。如：
Jim hurt his arm while [when, as] he was playing tennis.吉姆打网球时把手臂扭了。
As [When, While] she was waiting for the train, she became very impatient.她在等火车时，变得很不耐烦。
注意：as用于引出一个持续性动词表示“在……期间”时，其谓语通常只能是那些含有动作(action)和发展(development)意味的动词，一般不能是那些不用于进行时态的动词(如be, seem, love, want, agree, see, know, have等)，所以下面一句中的while不能换为as：
A：I'm going to the post office.我要去邮局。
B：While you are there, can you get me some stamps?当你在邮局时，能帮我买几张邮票吗?
2. “主长从长”型：即主句和从句为两个同时进行的动作或存在的状态，且强调主句动作或状态延续到从句所指的整个时间，此时通常要用while。如：
I always listen to the radio while I'm driving.我总是一边开车一边听收音机。
He didn't ask me in; he kept me standing at the door while he read the message.他没有让我进去，他只顾看那张条子，让我站在门口等着。
但是，若主句和从句所表示的两个同时进行的动作含有“一边……一边”之意时，则习惯上要用as。如：
He swung his arms as he walked.他走路时摆动着手臂。
I couldn't remember a story to tell the children, so I made one up as I went along.我想不出有什么故事可给孩子讲了，只好现编现讲。
3. “主长从短”型：即主句是一个持续性动作，而从句是一个短暂性动作，此时可以用as或when，但不能用while。如：
It was raining hard when [as] we arrived.我们到达时正下着大雨。
When [As] he came in, I was listening to the radio.他进来时，我在听收音机。
二、根据主句与从句动作是否同时发生来区分
1. 若主句与从句表示的是两个同时发生的短暂性动作，含有类似汉语“一……就”的意思，英语一般要用as (也可用when)。如：
The ice cracked as[when]I stepped onto it.我一踩冰就裂了。
He jumped to his feet as the boss came in.老板一进来他立刻站了起来。
但是，在hardly [scarcely]…when…句式中，不能将when换成as。如：
Scarcely had we arrived, when it began to rain.我们刚一到就下起雨来了。
2. 若主句与从句表示的是两个几乎同时发生的短暂性动作，含有类似汉语“刚要……就”“正要……却”的意思，英语一般要用as(也可用when)，且此时通常连用副词just。如：
I caught him just when [as] he was leaving the building.他正要离开大楼的时候，我把他截住了。
Just as [when] the two men were leaving, a message arrived.就在这两个人要离开的时候，突然有了消息。
三、根据是否具有伴随变化来区分
若要表示主句动作伴随从句动作同时发展变化，有类似汉语“随着”的意思，英语习惯上要用as，而不用when或while。如：
The room grew colder as the fire burnt down.随着炉火逐渐减弱，房间越来越冷。
As time goes by my memory seems to get worse.随着时间的流逝，我的记忆力似乎越来越差。
注：若不是引导从句，而是引出一个短语，则用with，不用as。如：
With winter coming on, it's time to buy warm clothes.随着冬天到来，该买暖和衣裳了。
The shadows lengthened with the approach of sunset.随着太阳下落，影子也逐渐伸长。
