happen to do和happen to be doing

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 未发生 happen to be doing 正在发生 happen to h 用happen to do和happen to sb.造句 happen to do sth 有没有happen to doing? happen with和happen to的区别 happen to和happen at的区别： 4,happen to do sth Do you happen to know____? Do you happen to know ( )?