happen与come about的区别

两个都做“发生”用的时候是没什么区别的。
只是come about还有“ 改变方向”的意思

不是的，得看具体句子。
How did this come about?这事是怎么发生的?
中this已经是主语，就不会用what.
what happened?中what就是主语。
你说Tell me what came about.也行的。
how can this happened?这怎么可能发生了？也行的。

(1)take place 一般指非偶然性事件的“发生”，即这种事件的发生一定有某种原因或事先的安排，例如：
Great changes have taken place in our hometown during the past ten years.
The Olympic Games of 2008 will take place in Beijing.

(2)happen一般用于偶然或突发性事件，例如：
What happened to you? (一般不说：What did you happen?)
Maybe something unexpected happened.
I happened to see him on my way home. = It happened that I saw him on my way home.

(3)come about表示“发生、产生”，多指事情已经发生了，但还不知道为什么，常用于疑问句和否定句，例如：
When Mother woke up, she didn’t know what had come about.
I’ll never understand how it came about that you were late three times a week.
Do you know how the air accident came about?