這三個詞的意思都有“當……時候”的意思。小編根據自己多年的經驗和例句，總結如下：
一、when 的用法(強調的是一個時間點)
如果只從現象來看，when 從句用的最多的是一般過去時，而主句的時態沒有限制，根據具體情況而定。
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.
他剛睡著電話鈴就響了。
When I met the Gills, I had been gardening for nearly ten years.
遇到吉爾一家時我已經做了近10年的園藝工作。
When he was a child he was always trying out new ideas.
他小時候就常常試驗一些新的設想。
when she came into my room I was just reading a book.
她走進我房間時，我正在看書。
Were you writing when the teacher came in?
老師進來的時候，你在寫信嗎？
Sorry，I was out when you called me.
對不起，你打電話來的時候我出去了。
He was on the point of leaving when someone knocked at the door.
他正要走,這時有人敲門。
I thought of it just when you opened your mouth.
就在你要說話的時候,我也想到了。
I had hardly[scarcely] closed my eyes when someone knocked at the door.
when 從句的謂語動詞可以在主句謂語動詞之前，之後或者同時發生， while, as從句的謂語動詞必須和主句的謂語動詞同時發生。
When he brought Imelda her drink she gave him a genuine, sweet smile of thanks
當他把飲料拿給艾梅爾達時，她真誠而甜美地向他微笑以示感謝。
在將來時從句中，常用when, 且要遵循主將從現的原則
例如：When my teacher comes here, I will meet her.
一閉上眼，就有人在敲門了。
根據以上的例句，我們可以總結出一點：when 從句的A事件，相當於另一個事件B發生的時間點。也就是說，when 從句的重點不在動作本身發生的狀態，而只是把它作為一個時間點，所以when 多數情況下用的是一般過去時，則不用正在進行時。因為如果用正在進行時，它表示的就是一段時間而不是一個時間點了。
實際上，when 從句也可以有其它的時態，但幾乎也不用進行時，因為它也只是作為一個時間參照點。例如：
When I got to the airport，the guests had left.
當我趕到飛機場時，客人們已經離開了。
When he had finished his homework，he took a short rest．
當他完成作業後，他休息了一會兒。
Why do you want a new job when you have got such a good one already？
你已經找到如此好的工作，為何還想再找新的？
You shall borrow the book when I have finished reading it.
在我讀完這本書後，你可以借閱。
When the manager comes here for a visit next week，Ill talk with him about this. 下週，經理來這參觀時，我會和他談談此事。
二、while 的用法（強調的是一段時間）
相比於when 來說，while 從句的側重點就不一樣了。while 從句的側重點在於描述動作正在發生的狀態，它的意思是：當while 事件正在發生的時候，另一件事如何如何。所以，while 從句一般用的是正在進行時。而另一件事的狀態沒有硬性的要求，根據具體情況而定。例如：
Don't interrupt him while he is thinking.
他在思考時別去打攪他。
She got married while working as a waitress.
她在飯店當服務生期間結婚了。
Pat killed time by watching TV while waiting for her.
帕特在等她時邊看電視邊打發時間。
While my wife was reading the newspaper, I was watching TV.
當妻子正在看報紙的時候，我正在看電視。
While Jim was mending his bike, Lin Tao came to see him.
正當吉姆修自行車時，林濤來看他。
While they were talking , the bell rang.
正在他們談話的時候，上課鈴響了。
You can't do your homework while you're watching TV.
你不能一邊看電視一邊做家庭作業。
While John was sitting biting his nails, I was working out a plan to get us home. 約翰坐在那裡咬指甲時,我想出了一個回家的辦法。
從時間的角度來看，while 表示的是一段時間，是一個過程。這是while 的側重點。因此，如果含有“一段時間”的含義的時候，就可以用while。例如：
Strike while the iron is hot. 趁熱打鐵。
這句話中，是說趁著鐵是熱的這段時間，趕緊打鐵。如果換成 when 意思就變了，相當於說鐵只熱了一下，打一下，然後鐵就冷了。這顯然不符合文意。再例：
—I'm going to the post office.
—While you're there, can you get me some stamps?
三、as 的用法（強調兩個動作同時發生，且表示持續時間不長）
1、as 從句表示的也是一件事情正在發生，另一件事也正在進行當中。但與 while 從句不同的是，as 從句用的一般不用正在進行時，而只是一般過去時。as 從句一般可以翻譯成“邊……邊……”。例如：
As my mother sang those old songs, tears ran down her cheeks.
當我媽媽唱起那些老歌時，眼淚順著她的臉頰流了下來。
The students took notes as they listened．
學生們邊聽課邊做筆記。
As we talked on, he got more and more excited.
我們繼續往下談的時候,他越來越興奮。
The little girls sang as they went．
小姑娘們一邊走，一邊唱。
Just as he caught the ball, there was a tearing sound.
當他抓住球的時候,有一種撕裂的聲音。
2、as 表達的事件，往往只是主句動作發生的背景或條件時，as 只是一個次要的時間說明，不像while 從句有強調 while 動作本身的意思。因此，as 常常翻譯成“隨著……”之意。 例如：
As the time went on，the weather got worse.
隨著時間的推移，氣候更加糟糕。
The atmosphere gets thinner and thinner as the height increases.
隨著高度的增加，大氣越來越稀薄。
As years go by，China is getting stronger and richer.
隨著時間一年一年過去，中國變得越來越富強了。
I caught a whiff of perfume as she swept past me.
她從我身邊走過時，我聞到一股香水味。
3、少數情況下，如果強調動作正在發生，as 從句也可以用正在進行時。這隻能算是特例了。
Joe jumped onto the bus just as it was pulling away.
正當公共汽車開動時，喬跳上了車。
The sad mother sat on the roadside，shouting as she was crying.傷心的媽媽坐在路邊，邊哭邊叫。
She was coming in as he was going out.
她進來的時候，他正要出去。
As we was going out, it began to snow.
我們出門時，開始下雪了。
He came in as I was going to bed.
正要上床睡覺，他進來了。
when 從句的謂語動詞可以在主句謂語動詞之前，之後或者同時發生， while, as從句的謂語動詞必須和主句的謂語動詞同時發生。
例如: When I had finished my homework, I took a rest.(finished先發生，這種情況只能用when)
Call me when you've finished.（只能用when）
你完成後就打電話給我。
When he brought Imelda her drink she gave him a genuine, sweet smile of thanks
當他把飲料拿給艾梅爾達時，她真誠而甜美地向他微笑以示感謝。（只能用when）
四、when, while, as 的互換
如果從句動作和主句動作同時發生，並且從句動作為延續性動詞時，when，while，as 可以互換使用。常見的延續性動詞有drink, eat, fly, have, keep, know, lie, live, keep, rain, read, run, sing, sleep, smoke, snow, stand, talk, wait, walk, wear, work, write, sit……這種情況下，它們的細微區別恐怕連英、美人自己也說不清了。
When /While /As we were dancing，a stranger came in.
當我們跳舞時，一位陌生人走了進來。 [dance 為延續性動詞]
When /While /As she was making a phonecall，I was writing a letter.
當她在打電話時，我正在寫信。 [make為延續性動詞]
While/When/ As I was walking down the street, I noticed a police car in front of the bank.
我順著馬路往前走時,發現銀行門前停著一輛警車。
知識拓展：
常用的表示時間的關聯詞
常用關聯詞：when, as, while, as soon as, while, before, after, since , once, till, until 等。
特殊關聯詞：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.
我直到我長大成人後才意識到我母親是多麼的與眾不同。
The children ran away from the orchard the moment they saw the guard.
孩子們一看見看守就跑出果園。
No sooner had I arrived home than it began to rain. 我剛到家就開始下雨了。
Every time I listen to your advice, I get into trouble.
每次我聽你的勸告，我就會陷入困境。
Until when can you stay?
你可以待到什麼時候？
‘I've got a new job.’ ‘Since when?’
“我有了份新工作。”“什麼時候開始的？
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