while,when,as 引导时间状语从句,尤其是过去进行时的区别.要简单易懂的,常用的.

合并下列句子,如果你会,你在这方面的问题就解决了：He was getting off the bus.He fell and hurt himself1 While he was getting off the bus,he fell and hurt himself2 When he fell and hurt himself,he was get...