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 .