19.8 比较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.
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。