分类：教育/科学-升学入学-高考
问题：谁能告诉我when,while,before,as的用法？尤其是 
尤其是before与的句型的对比．
回答：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。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 .回答完毕,希望能对你有帮助!