when__while__as的区别和用法(浓缩版)

When, as, while的使用区别
一、总体来讲, 三者的使用范围为when>as>while:
while的意思是“在……同时（at the same time that )”“在……期间(for as long as, during the time that）”。从while的本身词义来看，它只能表示一段时间(eg:进行时,be+表语)，不能表示具体的时间点(eg:一般时,将来时)。
1、He was watching TV while she was cooking.
2、He was waiting for me while I was working.
3、He asked me a question while I was speaking.
4、We should work hard while we are young.
While 后只能加进行时或be + 非名词表语(eg: While he was a boy, he lived in Japan.(×), 应将while改为when或as )
as的意思是“正当……时候（just as，at the same moment that）”“随着……(while，when)”,它既可表示一个具体的时间点，也可以表示一段时间。as可表示主句和从句的动作同时发生或同时持续，但不能表示两个动作一前一后发生。
1、As I got on the bus，he got off.
2、He was writing as I was reading.
3、The students were talking as the teacher came in.
大体来讲,as>while,即几乎能用while表时间的地方就能用as进行替换(除少数特殊情况), as≈且when的意思是“当……时候（at the time that）”“无论什么时候（whenever）” “一……就……（as soon as ）” “在……以后（after）”。它表示的时间概念比较广泛，上述例句中的as或while均可用when代替(除while中的例4)，因为when不但可以表示具体的时间点，而且也可以表示一段时间。
1、When he came in，she went out.
(此句中when从句的动作表示的是一个时间点，when可以用as代替，但不能用while代替。)
2、When he came back，I was doing some washing.
(此句中when从句的动作表示的是一个时间点，所以只能用as代替，不能用while代替。)
3. When Mary was writing a letter, the children were playing outside.
(此句中的when从句的动作表示的是一段时间，故可用as 或 while代替。)
When区别于while和as的最特殊性: 若主从句所表示的动作不是同时发生，而是有先后顺序时，要用 when.
1、When he had finished his homework，he took a short rest．（从句为完成时,finished先发生）
2、When I got to the airport，the guests had left．（主句为完成时, got to后发生）
3、I will go home when he comes back. (主句为将来时,comes后发生)
二、不遵循when>as>while规律的几种特殊情况(为了使表达的意思更地道,更精确,在一些特殊表达中最好选用特定词,而其它词的替换可能语法正确,意思有效,但不地道,不精准.)
只能用/最好用when的几种情况:
表示这时“这时(突然)”之意时
I was just about to go to bed when I heard a knock on the door. 我正要睡觉，听见有人敲门。
2. 表示“每当…的时候”(暗示一种规律性)
It’s cold when it snows. 下雪时天冷。
He smiles when you praise him. 你夸奖他时他总是笑笑。
只能用/最好用as的情况:
表示两个正在发展变化的情况，相当于汉语的“随着”
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 ( http: / / www.21cnjy.com / " \t "_blank )，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。)
表示的两个同时进行的动作, 含有“一边……一边”之意时
He swung his arms as he walked.
I couldn’t remember a story to tell the children, so I made one up as I went along.
表示一段时间的be结构 (as/ when +be + 名词)
As [When] a boy, he lived in Japan.
As [When] still a student, he wrote a novel.
4. 表示两个同时发生的短暂性动作，含有类似汉语“一……就”的意思
The ice cracked as [when] I stepped onto it. 我一踩冰就裂了。
He jumped to his feet as [when] the boss came in. 老板一进来他立刻站了起来。
(注：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。)
5. 表示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，且此时通常连用副词just。
I caught him just as [when] 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。
只能用/最好用while的情况:
表示“而”、“却”(表示对比), 也可以用作并列连词
One person may like to spend his vacation at the seashore, while another may prefer the mountains. 一个人会喜欢在海滨度假，而另一个人会喜欢在山里度假。
表示一段时间的be结构 (as/ when + 主语+be + 非名词[分词、介词短语、形容词])
While [When] reading, he fell asleep. 他看书时睡着了。
While [When] in trouble, ask her for help. 遇到麻烦的时候你就去找她帮忙。
You must study hard while (you are) young, or you will regret while (you are) old. 你趁年轻时必须努力学习，不然到老了你会后悔的。
表示的“当……的时候”具有“趁机”的意味
I tried to slip the note to him while [when] the teacher wasn’t looking. 我想趁老师不注意时把条子偷偷递给他。
He stole the money while [when] no one was by. 他趁旁边无人时把钱偷走了。
The boy looks at her repeatedly while [when] she’s not looking. 那男孩趁她没看他的时候不断地盯着她看。
Strike while the iron is hot. 趁热打铁。
4. 表示两个同时进行的持续性动作，且强调主句表示的动作延续到从句所指的整个时间(通常翻译成, 在……期间/时候, 某人一定(不)要, 一直(不)在, 总是(不)在做什么)
Don’t talk while you’re eating. 吃饭时不要说话。
I always listen to the radio while I’m driving. 我总是一边开车一边听收音机。
I kept silent while he was writing. 在他写的时候，我默不做声。
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。