1、 一般用when的情况，从句是短暂性动词
1）若从句是一个短暂性动作，而主句是一个持续性动作
When he came in， I was listening to the radio.
It was raining hard when we arrived.
2）若主、从句表示的是两个同时(或几乎同时)发生的短暂性动作
I thought of it just when you opened your mouth.
2、 一般用while的情况，若主、从句表示两个同时进行的持续性动作，且强调主句表示的动作延续到从句所指的整个时间。
Don't talk while you're eating.
I kept silent while he was writing.
3、 When和while可以互换的情况
He fell asleep when [while] he was reading.
I met him when [while] I was taking a walk in the park.
1，when
什么时候，何时什么情况下，什么场合下(用于时间的表达方式之后)在那时其时当时（副词）
什么时候，何时（代词）
在…时候，当…时，在…期间，在…之后在任何…时候（连词）
I had an active social life when I was at college.
我在大学时社交生活丰富。
Steve was in a particularly bad mood when he got back.
史蒂夫回来时心情特别不好。
When will he be back from his holiday?
他度假什么时候回来
I mind the time when we were in Beijing together.
我想起我们一起在北京的时光。
2，while
在…期间，当…的时候与…同时（连词）
虽然，尽管（连词）
(对比两件事物)…而，…然而（连词）
一段时间一会儿（名词）
Flats in the downtown are expensive， while houses in the suburbs are cheap.
市中心的公寓很贵，而郊区的房子很便宜。
While I sympathize， I can't really do very much to help.
虽然我很同情，但我实在帮不了什么忙。
Did you sneak into my room while I was asleep?
我睡着的时候你偷偷溜进我的房间了吗
The band played the guitar while we danced.
我们跳舞时乐队弹起了吉他。
While I was waiting at the bus stop， three buses went by in the opposite direction.
当我在公共汽车站等车时，有三辆公共汽车朝相反的方向经过。
I want to rest a while.
我想休息一会儿。
After a while， we naturally started talking about the children.
过了一会儿，我们自然而然地开始谈论孩子们。
They chattered away happily for a while.
他们愉快地聊了一会儿。
Life moves pretty fast. If you don't stop and look around once in a while， you could miss it.
人生如白驹过隙。如果你不偶尔停下来四处看看，你可能会错过许多。
