这三个词的意思都有“当……时候”的意思。小编根据自己多年的经验和例句，总结如下：
一、when 的用法(强调的是一个时间点)
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。
Sunday is the only day when I can relax.
星期日是我唯一可以休息的日子。
He had just drifted off to sleep when the phone rang.
他刚睡着电话铃就响了。
When I met the Gills, I had been gardening for nearly ten years.
遇到吉尔一家时我已经做了近10年的园艺工作。
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。
when she came into my room I was just reading a book.
她走进我房间时，我正在看书。
Were you writing when the teacher came in?
老师进来的时候，你在写信吗？
Sorry，I was out when you called me.
对不起，你打电话来的时候我出去了。
He was on the point of leaving when someone knocked at the door.
他正要走,这时有人敲门。
I thought of it just when you opened your mouth.
就在你要说话的时候,我也想到了。
I had hardly[scarcely] closed my eyes when someone knocked at the door.
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。
When he brought Imelda her drink she gave him a genuine, sweet smile of thanks
当他把饮料拿给艾梅尔达时，她真诚而甜美地向他微笑以示感谢。
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.
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。
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。例如：
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。
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。
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？
You shall borrow the book when I have finished reading it.
在我读完这本书后，你可以借阅。
When the manager comes here for a visit next week，Ill 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（强调的是一段时间）
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：
Don't interrupt him while he is thinking.
他在思考时别去打搅他。
She got married while working as a waitress.
她在饭店当服务生期间结婚了。
Pat killed time by watching TV while waiting for her.
帕特在等她时边看电视边打发时间。
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。
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。
While they were talking , the bell rang.
正在他们谈话的时候，上课铃响了。
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。
While John was sitting biting his nails, I was working out a plan to get us home. 约翰坐在那里咬指甲时,我想出了一个回家的办法。
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。例如：
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（强调两个动作同时发生，且表示持续时间不长）
1、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：
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。
2、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。 例如：
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。
I caught a whiff of perfume as she swept past me.
她从我身边走过时，我闻到一股香水味。
3、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。
Joe jumped onto the bus just as it was pulling away.
正当公共汽车开动时，乔跳上了车。
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。
She was coming in as he was going out.
她进来的时候，他正要出去。
As we was going out, it began to snow.
我们出门时，开始下雪了。
He came in as I was going to bed.
正要上床睡觉，他进来了。
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)
Call me when you've finished.（只能用when）
你完成后就打电话给我。
When he brought Imelda her drink she gave him a genuine, sweet smile of thanks
当他把饮料拿给艾梅尔达时，她真诚而甜美地向他微笑以示感谢。（只能用when）
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。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……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。
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]
When /While /As she was making a phonecall，I was writing a letter.
当她在打电话时，我正在写信。 [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。
知识拓展：
常用的表示时间的关联词
常用关联词：when, as, while, as soon as, while, before, after, since , once, till, until 等。
特殊关联词：the minute, the moment, the second, every time, the day，the instant, immediately , directly, instantly, no sooner … than, hardly …when, scarcely … when 等。
例句：
I didn't realize how special my mother was until I became an adult.
我直到我长大成人后才意识到我母亲是多么的与众不同。
The children ran away from the orchard the moment they saw the guard.
孩子们一看见看守就跑出果园。
No sooner had I arrived home than it began to rain. 我刚到家就开始下雨了。
Every time I listen to your advice, I get into trouble.
每次我听你的劝告，我就会陷入困境。
Until when can you stay?
你可以待到什么时候？
‘I've got a new job.’ ‘Since when?’
“我有了份新工作。”“什么时候开始的？
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