when, while, as的区别
一、as引导从句时侧重主从句动作同时或几乎同时进行，从句的动作可以是持续性的，也可以是短暂的。

如：  
Sometimes I watch TV as I am having breakfast．  

二、when和while还可以是并列连词，意思分别是“就在这时”，“然而”。

如：  
I was having a rest on the sofa when the telephone rang．  
They were surprised that a child should work out the problem while they couldn’t .  

三、while侧重主从句动作的对比，且从句的动词必须是持续性的。

如：  
While we were chatting she was looking at the time table on the wall．

 
四、when引导从句时，主从句的动作有先有后，也可以同时进行,从句的动作可以是持续性的，也可以是短暂的。

如：  
When I got to the airport, the plane had already taken off. （主先从后）（短暂性）  
When I lived there, I used to go to the seaside on Sundays. (同时) （持续性）  
When the movie ended, the people went back. （从先主后）