happen to do和happen to be doing

问题描述:

happen to do和happen to be doing
有什么区别?区别是?
HAPPEN TO BE DOING加的是进行时是吗?那是不是不能加结束性动词(短暂动词)了

happen to do ,碰巧做了.happen to doing 碰巧正在做,是,可以