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

用happen to do和happen to sb.造句 happen to do or happen doing? happen to和happen at的区别： happen with和happen to的区别 问个关于happen的用法的问题.happen.不是happen to do sth.那么 碰巧去做,碰巧正做,和碰巧已做.是不是分别是happen to do,happen to doing,happen to have been? happen to do和happen to be doing 的区别?顺便分别解释下.我问的是happen to be doing 不是happen to doing~ happened to do和‘happen to do ;区别? happen to sb和happen to do sth 造句并解释造的句子 sb.happen to do sth. happen to happen to happen to happen to ,happen 怎么用? happen on 和 happen to 有什么区别正确. happen to do sth什么意思 chance/happen/seem/appear/pretend to do happen to do sth的汉语是什么? happen 有 sth happen to sb或者sb happen to do sth 那么occur同样的用法吗?