【It （so） happens that 碰巧,恰巧】为什么happen前能加so?

【It （so） happens that 碰巧,恰巧】为什么happen前能加so? It so happens that we often see a notice on a door to a conf 这题为何选It so happen It is so unbelieveable that such a murder ____(happen) in th it seems that it happens that 和seem,happen相同用法 It (so) happened that.什么时候加so? 同义句转换原句：Everything seemed to happen so fast）（?）that everythi So that explains it everything . So that's how it it's so happened that I was wrong,so it won't happen again 关于it happens that