when, while和as的用法：
（1）三者均可表示“當……的時候”,如果主句謂語動詞表示的是短暫的動作,而從句表示的是一段時間,三者可以通用.例如：
I met an old friend of mine as / when / while I was walking along the street. （當我沿街散步時,碰見了我的一個老朋友.）
（2）as和when引導的時間狀語從句中,謂語動詞可以是終止性動詞,while引導的時間狀語從句中,謂語動詞一般是延續(xù)性動詞.例如：
It was raining hard when we got to the airport. （當我們到達機場時,天正下著大雨.）（不能用while 引導）
（3）as強調主句謂語動詞與從句謂語動詞表示的動作同時發(fā)生；while強調主句謂語動詞表示的動作持續(xù)于while從句所指的整個時間內；when可以指主句謂語動詞和從句謂語動詞所表示的動作同時或先后發(fā)生.例如：
She sang as she went along. （她邊走邊唱.）
Please listen to me carefully while I read. （我朗讀的時候,請仔細聽.）
When he reached home, he had a little rest. （回到家以后,他休息了一會兒.）
before的用法與前面三個詞區(qū)別較大,具體如下:
1. 表示“在…..之前就…..”.
Mary had finished her homework before her mother returned.
2. 表示“過了多久后才…….. ,動作進行到什么程度才……”
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“來不及； 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“剛…就…”
I had hardly sat down before the bell rang.
5. 表示“不知不覺就…..； 還沒弄清就……”
Time passed quickly and three months went by before Aqiao knew it.
6. 表示“寧愿…,決不…,與其…,毋寧….”常與will 或would 連用.
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“否則；以免”
Do it now before you forget . I will write it down before I forget it .
8. 用于句型“it was +時間段+ before …” 表示 “過了多久才…”
It was three days before he came back. .
9. 用于句型 “ it was not long +時間段+ before …” 意為“不久就….”
It was not long before he sensed the danger of the position .
10. 用于句型“it will be(not )+時間段+ before….”表示“要過多久/ 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 .