while与when作为"当……时候"讲时有什么区别?

下面是老师的答复:
1. 当从句中的谓语动词是持续性的动作时,我们可以用while, when或as.
While / When / As I was walking down the street, I noticed a police car in front of the bank.我顺着马路往前走时,发现银行门前停着一辆警车.
2. 当从句中的谓语动词表示瞬间动作时,不可用while.
He was on the point of leaving when someone knocked at the door. 他正要走,这时有人敲门.
3. 如果谈论两个长动作,最常用的是while.
While John was sitting biting his nails, I was working out a plan to get us home. 约翰坐在那里咬指甲时,我想出了一个回家的办法.
不过, as是可以用来说明两种正在发展或变化的情况的,这时as引导一个持续性的动作,主句中的动作与之同时进行.
As we talked on, he got more and more excited. 我们继续往下谈的时候,他越来越兴奋.

while 后应该加 一段时间
when 后可加 一段或一点时间
He came in while I was eating
I was eating when he came in