when, while , as的用法

when当(在....时: ( = at the tine that), 可表示瞬间、时间

段,与主从句所述动作、事情可同时,也可有先后:It was snowing when we got to the airport.

我们到达机场时,天正下着雪。(同时发生)

He went horne when he had finished his homework.做完作业后他回家去了。(先后发生)

while正当(正在...时;(指同时=①for as long as②during the time that),用于同时进行的两延续性动词相伴随而发生,常对同类的两动作进行对比:

Please write while I read. ( = during the time that )我念的时候,请写下来。

I am safe while l am here.(=for as long as)只要我在这儿，我就安全。

Some students were reading while others were. writing.有些学生在读,而有些学生在写。( 进行对比)

as当....时;强调at the same time,

①不指先后,而指并相发生，尤指短动作或事件同时发生:

As I looked, some one came near.

正当我看的时候,有个人走上前来。

②as还可说明两种正在发展或变化的情况:As I get older I get more optimistic.

我越活越乐观。

as,when或while可引导时间较长的“背景”情况:

As ( When,或While) I was walking down the street, I noticed apolice car in front of Number 37.

我顺着马路往前走,发现37号门前停着- .辆警车。