when 与 while 的区别
____表示"当……时候"，既指时间点，又指一段时间，when 引导的时间状语从句中的动词可以是终止性的也可以是延续性的。
When the teacher came in, the students were talking.
When she arrives, I'll call you.
____表示"当……时候"，仅指一段时间，从句中的动作必须是延续性的，一般强调主从句的动作同时发生， while还可以作并列连词，意为"而、却"，表示对比关系。
Lisa was singing while her mother was playing the piano.
Tom is strong while his younger brother is week.
