while, when, as
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.
As the day went on, the weather got worse.