1）as, when 引導短暫性動作的動詞。
just as / just when / when i stopped my car, a man came up to me.
2）當從句的動作發生於主句動作之前，只能用when 引導這個從句，不可用as 或 while。
when you have finished your work, you may have a rest.
3）從句表示"隨時間推移"連詞能用as，不用when 或while。
as the day went on, the weather got worse.
日子一天天過去，天氣越變越壞。