1、as 在-期间，当-的时候”引导时间状语从句.注意与 when、 while 的用法区别。 下列情形时,只用 as, 而不用 when 或 while。 1)用于表示同一个人的两种动作交替进行,指一边-一边. 如: The girl sings as she goes to school. He looked behind occasionally as he went forward in the forest. 2)表示两个同步发生的动作或行为,意思是随着-的发展 .如: As time went on / by, she became more and more beautiful .
2、As children get older, they become more and more interested in everything. 3)表示两个短暂行为或事情几乎同时发生.如:I watched her as she read the book. I thought of it just as you opened your mouth. Just as the flying worm hit her face, she gave a loud cry. 4)接在名词后面表示某一个年龄段时.如: As a young man, he was active in sports.
3、 when 作“当的时候“解,可以指较短的(一点) 时间,也可指一段时间。如 :When I was at college, I could speak several foreign languages. When the clock struck twelve, I had slept for 2hours. while 常表示一段较长的时间或一个过程,强调主句谓语动词与从句谓语动词同时发生。如:Work while you work. Play while you play.while 引导状语从句时，应注意以下几点：1. while 引导的时间状语从句既可放在句首，也可放在句末；而 w
4、hile 引导的让步状语从句一般放在句首，不放在句末。另外，也可以从意义上加以区分。例如：While he was tired ， he went on working.（虽然他累了，但他还是继续工作。 ）（ while 引导让步状语从句）While he was tired ， he would have a rest.（他累了就休息一会儿。 ）（ while 引导时间状语从句）2. while 引导让步状语从句时，如果从句的主语和主句的主语相同，那么从句的主语和谓语动词 be 都可以省略。例如：While he was ill ， he went there. （= While ill，
5、he went there.）（虽然他有病，但他还是去那里了。 ）3. while 和 when 都可以引导时间状语从句，但应注意： while 引导的时间状语从句中的谓语动词必须是延续性的动词；而 when 引导的时间状语从句中的谓语动词既可以是延续性动词，也可以是非延续性动词。当时间状语从句中的谓语动词是延续性动词时， while 和 when 可以互相替换。例如：While we were talking， the teacher came in.（我们正在谈话时，老师进来了。 ）The bus had left when we arrived at the bus stop.（当我们到
6、达车站时，公共汽车已经开走了。 ）（ arrive 是非延续性动词，因此，此句中的 when 不能用 while 替换。 ）When / While we were having a meeting， our teacher came in.（我们正在开会时，老师走了进来。 ）（ have 是延续性动词，因此，句中的 when 可以用 while 替换。 ）4. while 引导的状语从句还可以表示“某个动作正在进行的同时，另一个动作也在进行之中”。例如：While the teacher was speaking， the students were listening.（老师讲课时，学生们在听。 ）While he was writing， I was reading.（他写字时，我在看书。 ）
