While 和When有什么区别?跟过去进行时相关的题,

题目

While 和When有什么区别?跟过去进行时相关的题,
1Tim was riding his bicycle ——he saw his thecher.2.e arrived in Changchun ——it was snowing.
如果用了When,还能不能用Ing形式V?举个例子

答案