1）as, when 引导短暂性动作的动词。例如：
Just as / Just when / When I stopped my car, a man came up to me. 我一刹车，有一个人向我走来。
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。例如：
When you have finished your work, you may have a rest. 赶完活后，你可以休息一下。
3）从句表示"随时间推移"连词能用as，不用when 或while。例如：
As the day went on, the weather got worse. 日子一天天过去，天气越变越坏。