happen to 未发生 happen to be doing 正在发生 happen to h

happen to 未发生 happen to be doing 正在发生 happen to h happen to do和happen to be doing happen to do和happen to be doing 的区别? happen to do or happen doing? happen to 有没有happen to doing? happen to ,happen 怎么用? occur to /happen to happen with和happen to的区别 happen to和happen at的区别： happen to sb 意思 what happen to sb.