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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 .
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