while表示一段时间,但不会很长.
比如,he was watching TV while i was reading.
when表示一个时间点,常表示二个动作（或者状态）的前后.
比如,your question had been solved out when i started to answer it.
故去分词用于过去时
比如,He is retired.他已退休.
过去进行时表示的是过去正在进行的动作,或者呈现的状态
比如:
My mother was cooking when i returned to home.
It was raining when i came out of the school