happen on后面加的可能是时间/地点等名词（短语），而这个短语恰好是要求用介词on的。
比如：What happend on the island? 这个岛上发生什么事？
A traffic accident happened on July 12th? 7月12日发生了一起交通事故。
happen on不是固定搭配。
happen to有两种理解。
sth happen to sb/sth 某人/某物发生某事
如：What happend to you? 你怎么了？/你出什么事了？
sb/sth happen to do sth 某人/某物碰巧做某事
如：I happened to meet your father yesterday. 我昨天刚好碰到你爸爸了。
happen to是固定搭配。
happen ['hæpən]
vt
引起，发生:
Accidents will happen.
很有可能会发生意外。
to happen what may
无论发生什么情况
降临(到某人身上)(常与to连用):
Please give it to my wife if anything should happen to me.
万一我遇到不幸，请把这个交给我妻子。
碰巧；恰好:
Do you happen to remember his car number?
你还记得他的车牌号码吗?
It so happens that I'm free today.
真是巧极了，正好我今天有空。
偶然遇见；偶然发现(常与on或upon连用):
He happened on a clue to the mystery.
他偶然发现了那秘密的线索。
[主美国英语]偶然出现；碰巧来(或去):
My friends happened along at that time.
那时我的几个朋友碰巧来了。
I bought today's newspaper for you as I happened past the post office.
我刚巧路过邮局，就给你买了今天的报纸。
[俚语]极其兴奋，非常有趣
adv.
[方言]偶然，碰巧
近义词:
chance . occur . transpire
短语:
as it happens 碰巧；偶然
happen in with 偶然遇到
happen what may (或will) 不管怎样，不管发生什么情况
It (so) happens that 碰巧，恰巧
词组短语同近义词同根词词语辨析
happen to be 碰巧是；恰巧是
as it happens 碰巧；偶然发生
happen on 偶然遇到或发现
happen to meet 偶然遇见
happen to coincide 不约而同；不谋而合
双语例句原声例句权威例句
We do not want that to happen.
我们不希望发生这样的事。
www.voanews.cn
I dread to think what will happen next.
我不愿想下一次会发生什么。
《21世纪大英汉词典》
These events were clearly predestined to happen.
这些事件是不可避免要发生的。
词组短语
happen to be 碰巧是；恰巧是
as it happens 碰巧；偶然发生
happen on 偶然遇到或发现
happen to meet 偶然遇见
happen to coincide 不约而同；不谋而合