用表示时间的连词连接一个句子作状语，这样的主从复合句就是时间状语从句。连接时间状语从句的连接词有：when, before, after, while, as soon as, until, since等等，那么when和while及as引导时间状语从句的区别有哪些呢？
1.when引导的从句的谓语动词可以是延续性的动词，可以是瞬时动词。并且when有时表示“就在那时”。
例如：
When she came in, I stopped eating.她进来时，我停止吃饭。
When I lived in the countryside, I used to carry some water for him.当我住在农村时，我常常为他担水。
We were about to leave when he came in.我们就要离开，就在那时他进来了。
2.While引导的从句的谓语动作必须是延续性的，并强调主句和从句的动作同时发生（或者相对应）。并且while有时还可以表示对比。
例如：While my wife was reading the newspaper, I was watching TV.
I like playing football while you like playing basketball.我喜欢踢足球，而你喜欢打篮球。
3.As表示“一边……一边”，as引导的动作是延续性的动作，一般用于主句和从句动作同时发生；as也可以强调“一先一后。
例如：
We always sing as we walk.我们总是边走边唱。（as表示“一边……一边”）
As we were going out, it began to snow.当我们出门时，开始下雪了。
