一．when, while, as 都有“当...的时候”的意思, 具体用法如下：
When/As:
e.g: He was sleeping when/as the film started.
e.g: My father came into my room when/as I was watching TV.
总结：1.主句（过去进行时）+ when/as + 从句（过去时） 动词:主长从短
2. 主句（过去时）+ when/as + 从句（过去进行时） 动词:主短从长
While:
e.g: He was sleeping while his father was reading.
e.g: My father came into my room while I was watching TV.
总结：1.主句（过去进行时）+ while + 从句（过去进行时） 动词:主长从长
2. 主句（过去时）+ while + 从句（过去进行时） 动词:主短从长
指某一段时间发生的事情，while, when, as均可用
指某一时间点发生的事情，只能用when, as
while和as 引导的时间状语从句，主从句谓语动作同时发生
总结：主长从长用while，主长从短用when/as，主短从长三者皆可，
二．When, while, as 表达其他意思的情况。
e.g When will you finish your homework? 何时...
e.g My brother likes watching TV while I like doing sports. 然而...
e.g As the temperature drops, winter will come soon. 随着...
e.g I choose the subjects the same as you. 正如...
e.g The boy walked as he sang. 一边...一边...
e.g As a student, we should study carefully. 作为...