when,as,while這三個詞都可以引出時間狀語從句,
它們的差別是：
when 從句表示某時刻或一段時間
as 從句表示進展過程,
while 只表示一段時間
When he left the house,I was sitting in the garden.
當他離開家時,我正在院子里坐著.
When he arrived home,it was just nine o'clock.
當他到家時,正是九點鐘.
As he grew older,he became more intelligent.
隨著他年齡的增長,他變得更有才智了.
While he was in London,he studied music.
當他在倫敦時,他學習音樂.
While she was typing the letter,the telephone rang.
當她在用打字機打那封信時,電話響了.