when,as,while這三個(gè)詞都可以引出時(shí)間狀語(yǔ)從句,它們的差別是：when 從句表示某時(shí)刻或一段時(shí)間,有“當(dāng)……時(shí)”的意思；
as 從句表示進(jìn)展過(guò)程,有“隨著,一邊……一邊”的意思；
while 只表示一段持續(xù)的時(shí)間,有“在……的同時(shí)”的意思.
例如：
When he arrived home,it was just nine o'clock.
當(dāng)他到家時(shí),正是九點(diǎn)鐘.
As he grew older,he became more intelligent.
隨著他年齡的增長(zhǎng),他變得更有才智了.
While she was typing the letter,the telephone rang.
當(dāng)她在用打字機(jī)打那封信時(shí),電話響了.when while都可以表示一段時(shí)間，while一般接的是比較持續(xù)性的動(dòng)作；表示“當(dāng)...的時(shí)候”when while一般放在句首，放在句中或句尾的while含有比較的意思，比如： Father was watching TV,while mother was cooking.