happens

happen to sb, happen with sb

这两者都是“某人发生了某事”的意思,前者可指坏事,也可指好事; 而后者多指坏事。试比较:
What's happened to her?
她怎么了?
What's happened with her?
她出什么事了?

happen, occur, take place

这三者都可作“发生”解。它们的区别是:
1.happen和occur往往表示事情的发生带有偶然性, happen是一般用语, occur是较正式用语; take place常指按计划“发生”。例如:
The explosion happened〔occurred〕 during an exercise near the town.在离市镇不远的地方演习时发生了爆炸。
The talk is scheduled to take place tomorrow morning at eight o'clock.预定明早8时举行会谈。
2.当表示具体事情的名词作主语时,用happen和occur都可,如果不是表示具体事情的词则多用happen。在否定句里一般用occur。例如:
These events happened〔occured〕 last year.这些事是去年发生的。
What happened after I left?我走后发生了什么事情?
It didn't occur to him to ask for help.他没想到请别人帮忙。

下面两个句子意思不同:

It happens that I am there.
碰巧我在那里。
It so happens that I am there.
事情发生得那么凑巧,我在那里。
第1句中的it是形式主语,第2句中的it是第三人称代词,相当于the thing。