happen 后边跟to do为什么不是to doing?happen为不及物动词 to应该是介词所以我认为应该是to

1 sth happens to sb.-----某人发生了某件事An accident happened to him 他出事了---to是介词
2 sb.happens to do sth（碰巧- - -- ）.----- 如：I happen to know him.----我碰巧认识他-----to是动词不定式的连接词