He was riding to school -- he was hit by a car this morning.

when可以表延续或非延续性~while只表延续性
when都可以填~而while只能在延续性句子里出现!
He was riding to school __ he was hit by a car this morning
这句话是过去式~是没有延续!就是非延续~所以必须填when!
以后你都填when就绝对不会错!when延续或非延续性都可以表示!
我们也才学完!