1）as, when 引导短暂性动作的动词。
　　　　　 Just as / Just when / When I stopped my car, a man came up to me.
　　2）当从句的动作发生于主句动作之前，只能用when 引导这个从句，不可用as 或 while。
　　　　　 When you have finished your work, you may have a rest.
　　3）从句表示随时间推移连词能用as，不用when 或while。
　　　　　 As the day went on, the weather got worse.　
　　　　　日子一天天过去，天气越变越坏。