while可以放于句首,引导时间状语从句,其后的动词一般使用延续性动词,如：While I was reading,my father came in.
when也可以位于句首,同样引导时间状语从句,但其后的动词不一定是要用延续性的动词.如：when I can't do my homework,i will ask for my sister./ When he reached home,I was watching TV.