while和when 有什么区别．

when可表示时间的一先一后（=after）
e.g. When he has done his homework, he watches TV.
I am talking with her when he comes in.(when 放当中表suddenly突然发生)
when表持续时间：It was snowing when he arrived at station.
while表一段时间：While he was reading, I was playing the piano.
表示两个同时发生的事件，如果都有延续性，而且延续时间大致相等，连词可用when,while。
e.g. The wind blew hard when the rain poured down.
While I read, she rang.
I was cooking the dinner while he was playing the piano.

可以注意到一般情况下，都作“当……的时候”的意思时，while后面加现在进行式， when则是前面用现在进行式

参考文献：新编英语语法