happen表示“碰巧或恰巧发生某事”时,可用“It happens / happened that...”这一结构来表达.例如：
It happened that Brian and Peter were at home that day.碰巧那天布莱恩和彼得在家了.
It happened that he had to take part in a meeting that afternoon.碰巧那天下午他不得不参加一个会议.
so只是个副词,为了表明事情发生的偶然程度,就是说,在表达“太巧了……”的时候,加so,加了so的时候,that后面的从句通常强调的是“如此巧合”造成的“结果”.
“There happened to be nobody in the house on fire.”中,happened to 依然表示“碰巧”的意思,但是个用在“There be”句型中的独立成分.
“It (so) happened that.”之所以用“it”,是因为that后面是从句,it做的是that后从句的形式主语!
如果把“There happened to be nobody in the house on fire.”这句改成从句形式,那就是“It happened that there was nobody in the house on fire.”