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。

表示"一…就…"的结构

hardly/scarcely…when/before, no sooner…than　和as soon as都可以表示"一…就…"的意思，例：

I had hardly / scarcely got home when it began to rain.
I had no sooner got home than it began to rain.
As soon as I got home, it began to rain.

注意：如果hardly, scarcely 或no sooner置于句首，句子必须用倒装结构：
Hardly / Scarcely had I got home when it began to rain.
No sooner had I got home than it began to rain.