when,as,while这三个词都可以引出时间状语从句,
它们的差别是：
when 从句表示某时刻或一段时间
as 从句表示进展过程,
while 只表示一段时间
When he left the house,I was sitting in the garden.
当他离开家时,我正在院子里坐着.
When he arrived home,it was just nine o'clock.
当他到家时,正是九点钟.
As he grew older,he became more intelligent.
随着他年龄的增长,他变得更有才智了.
While he was in London,he studied music.
当他在伦敦时,他学习音乐.
While she was typing the letter,the telephone rang.
当她在用打字机打那封信时,电话响了.