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Were you writing when the teacher came in?
老师进来的时候，你在写信吗？
When he was a child, he was always trying out new ideas.
他小时候就常常试验一些新的设想。
(二) while只能表示某一段时间，不表示某一点时间。在while引导的时间状语从句中，其谓语动词只能是延续性的，而且也只能与主句中的谓语动词同时发生或存在。如：
While Jim was mending his bike, Lin Tao came to see him.
正当吉姆修自行车时，林涛来看他。
You can’t do your homework while you’re watching TV.
你不能一边看电视一边做家庭作业。
(三) when和while的区别还在于： while引导的时间状语从句多用进行时态，而when引导的时间状语从句多用一般时态。如：
While they were talking, the bell rang.
他们谈话的时候，上课铃响了。
I was doing my homework when my mother came back home yesterday evening.
昨天晚上妈妈回家的时候，我正在做家庭作。
(四) when可表示“这时(突然)” ；while表示“而”“却”(表示对比) 。如：
I was just about to go to bed when I heard a knock on the door.
我正要睡觉，听见有人敲门。
He likes coffee, while she likes tea.
他喜欢咖啡，而她却喜欢茶。
1 He was listening to music ______ he was riding his bike.
2 I was waiting at the lights ______ I saw the boy fall off his bike.
3 I was walking in the street ______ an accident happened.
4 The driver was talking on his mobile phone ______ he was driving.
while
Complete the sentences with when or while.
when
when
while
What were they doing when it began to rain?
They were playing computer games when it began to rain.