when 后边加ing的时候?什么时候?while 能加原型吗?

当主语与主句主语相同时,可以 ,这属于省略问题.
e.g.When listening for statistics,you may hear percentages(百分数).
When listening =When you listen
while不能加原形,但可以加v-ing形式
e.g.While crossing the street,you must be careful.