happen to和happen at的区别：

happen to sb 某人怎么了 What happened to you?你怎么了？
happen to do 碰巧怎么了 I happened to see my teacher in the street.我在街上碰巧看到老师了。
happen at 不是固定用法，at 可接时间点或地点

happen to是“发生在····”的意思，happen at是“偶然”的意思

happen to 后面跟的是人名、公司、或其他集合名次和代名词
What happened to you?
It happens to everybody.
The same thing happened to John and his family last week.
Please make sure this doesn't happen to us again.
happen at 后面跟的是时间、地点等状词,补充说明发生的那件事情.
The accident happened at near the corner of Main Street and Hill Avenue.
The recent earthquake in Janpan happened at 1:48pm the day before yesterday.