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)當從句的動作發生于主句動作之前，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。