while as 的区别?while as 这两个表虽然时的区别究竟是什么?

when,while这三个词都有"当……时候"之意,但用法有所不同,使用时要特别注意.
①when意为"在……时刻或时期",它可兼指"时间点"与"时间段",所引导的从句的动词既可以是终止性动词,也可是持续性动词.如：
When I got home,he was having supper.我到家时,他正在吃饭.
When I was young,I liked dancing.我年轻时喜欢跳舞.
②while只指"时间段",不指"时间点",从句的动词只限于持续性动词.如：
While I slept,a thief broke in.在我睡觉时,盗贼闯了进来.
辨析
①when从句与主句动作先后发生时,不能与while互换.如：
When he has finished his work,he takes a short rest.每当他做完工作后,总要稍稍休息一下.(when = after)
When I got to the cinema,the film had already begun.当我到电影院时,电影已经开始了.(when=before)
②when从句动词为终止性动词时,不能由while替换.如：
When he came yesterday,we were playing basketball.昨天他来时,我们正在打篮球.
③当从句的谓语是表动作的延续性动词时,when,while才有可能互相替代.如：
While / When we were still laughing,the teacher came in.正当我们仍在大声嬉笑时,老师进来了.
④当从句的谓语动词是终止性动词,而且主句的谓语动词也是终止性动词时,when可和as通用,而且用as比用when在时间上更为紧凑,有"正当这时"的含义.如：
He came just as (or when) I reached the door.我刚到门那儿,他就来了.
⑤从句的谓语动词如表示状态时,通常用while.如：
We must strike while the iron is hot.我们应该趁热打铁.
⑥while和when都可以用作并列连词.
Mike is tall while his brother is short.迈克的个子很高,然而他弟弟很矮.
Mary was on her way home when two big boys stopped her.玛丽正走在回家的路上,这时两个大个子男孩拦住她.

when, while和as的用法：
（1）三者均可表示“当……的时候”,如果主句谓语动词表示的是短暂的动作,而从句表示的是一段时间,三者可以通用.例如：
I met an old friend of mine as / when / while I was walking along the street. （当我沿街散步时,碰见了我的一个老朋友.）
（2）as和when引导的时间状语从句中,谓语动词可以是终止性动词,while引导的时间状语从句中,谓语动词一般是延续性动词.例如：
It was raining hard when we got to the airport. （当我们到达机场时,天正下着大雨.）（不能用while 引导）
（3）as强调主句谓语动词与从句谓语动词表示的动作同时发生；while强调主句谓语动词表示的动作持续于while从句所指的整个时间内；when可以指主句谓语动词和从句谓语动词所表示的动作同时或先后发生.例如：
She sang as she went along. （她边走边唱.）
Please listen to me carefully while I read. （我朗读的时候,请仔细听.）
When he reached home, he had a little rest. （回到家以后,他休息了一会儿.）
before的用法与前面三个词区别较大,具体如下:
1. 表示“在…..之前就…..”.
Mary had finished her homework before her mother returned.
2. 表示“过了多久后才…….. ,动作进行到什么程度才……”
The man almost knocked me down before he saw me .
They walked about 30 miles to the west before they saw a village .
They worked day and night about three days before everything returned to normal .
3. 表示“来不及； 尚未…..就…”
Before I could get in a word , he had measured me .
He ran off before I could stop him .
She left the company before I could have a word with her .
4. 表示“刚…就…”
I had hardly sat down before the bell rang.
5. 表示“不知不觉就…..； 还没弄清就……”
Time passed quickly and three months went by before Aqiao knew it.
6. 表示“宁愿…,决不…,与其…,毋宁….”常与will 或would 连用.
I would die before I would tell
I’d shoot myself before I apologized to him.
I would / will die before I surrender .
7. 表示“否则；以免”
Do it now before you forget . I will write it down before I forget it .
8. 用于句型“it was +时间段+ before …” 表示 “过了多久才…”
It was three days before he came back. .
9. 用于句型 “ it was not long +时间段+ before …” 意为“不久就….”
It was not long before he sensed the danger of the position .
10. 用于句型“it will be(not )+时间段+ before….”表示“要过多久/ 不久…才…”
It will be half a year before you graduate from this school .
It will be not long before you find you are wrong .

when,while和as的用法区别
　　①三者均可表示“当……的时候”,如果主句表示的是短暂的动作,而从句表示的是一段时间,三者可通用.如：
I met Kang Li as／when／while I was walking along the street．当我沿街散步时碰见了康丽.
　　②when可以和延续性动词连用,也可以和短暂性动词连用；而while和as只能和延续性动词连用.如：
It was snowing when we got to the airport．当我们到达机场时,天正下着雪.（不能用while）
　　③as强调主句与从句表示的动作同时发生,as常表示“随着……”；“一边……,一边……；while强调主句表示的动作持续于while所指的整个时间内；when可指主、从句所述动作同时或先后发生.如：
As the time went on,the weather got worse．（as表示“随着……”之意）
He sang as he went along．他边走边唱.
Please write while I read．我读的时候,请写下来.
When he reached home,he had a little rest．回到家后,他休息了一会儿.
④when用于表示“一……就……”的句型中（指过去的事情）.
somebody had hardly（＝scarcely） done …when．．．＝Hardly ／ Scarcely had somebody done …when．．．
①I had hardly ／scarcely closed my eyes when someone knocked at the door．＝Hardly ／ Scarcely had I closed my eyes when someone knocked at the door．我刚一闭上眼,就有人在敲门了.