When的范围i比较大,可以指接延续和非延续动词,When i got there ,he had gone.
When i was walking on the street ,i met an old friend.
此外when 还可以“就在那时” I was walking in the forest when a lion appeared in front of me.
While 所引导的时间状语从句必须谓语动词是延续的,并且强调在此期间.如 While he was reading ,the electricity was cut off.
As 所引导的时间状语从句主要强调“随着”或“一边.一边”如：As time goes by ,i love you more and more.As they were walking ,they were singing.