when, while和as引导时间状语从句用法辨析
三者皆可引导时间状语从句，但用法有区别。
Ⅰ. as, when和while都可表示同时发生的事。如：
As/When/While I was walking down the street, I noticed a police car in front of that hotel.
Ⅱ. when可表示从句的动作与主句的动作同时发生，或先于主句的动作。while仅仅表示从句的动作与主句的动作同时发生，并且有延续的含义。因此，该从句所用的谓语动词常常是延续性的。如：
When he had finished his homework, he took a short rest.
Strike while the iron is hot.
Ⅲ. 从时间上说，when可指一段时间，也可指某一时间点；while仅可指一段时间，它引导的从句中只能使用延续性动词，而不是非延续性动词。as主要是指一段时间，但有时和when一样也指某一时间点。如：
When/As Jim arrived, I was cooking my supper.
While the child was playing his toys, his parents were watching TV.
When I got to the theater, I found that all the tickets had been sold out.
Ⅳ. 在下列情况下只能用when
1. 用在一般现在时代替一般将来时、一般过去时代替过去将来时的时间状语从句中。如：
When the plants are in the ground, I’ll put some powder on the soil.
2. when用在并列分句中，意为then, at that time。如：
I was about to tell him what had happened when the telephone rang.
[小试身手]
Ⅰ. 用when, while和as填空。
1. I woke up, Mom was preparing breakfast.
2. I was about to leave the telephone rang.
3. The students took notes they listened.
4. we were dancing, a stranger came in.
5. years go by, China is getting stronger and richer．
Ⅱ. 单项填空。
1. he has finished his work, he takes a short rest.
A. When B. While
C. Whether D. Before
2. the time went on, the weather got worse.
A. When B. As
C. While D. Since
3. I couldn’t remember a story to tell the children, so I made one up I went along.
A. since B. while
C. as soon as D. as
4. — Did Jack come back early last night?
— Yes. It was not yet eight o’clock he arrived home.
A. when B. that
C. while D. until
5. This afternoon I was just about to go swimming luckily our guide saw me and stopped me.
A. when B. while
C. as D. until
参考答案：
Ⅰ. 1. When 2. when 3. as
4. When/While/As 5. As
Ⅱ. 1-5 ABDAA |
