I don't know how the accident happened.当I don't know how the accident -- -- --.该用哪些词

occured, took place， came about， came into being 都是发生的意思

I don't know how the accident took place.
I don't know how the accident occurred.
I don't know how the accident existed.

I don't know how the accident 两个空就是:took place/came about /三个空就是:came into existence 发生 take place…; come about; occurrence; bringabout; When did the world come into existence 世界是什么...

happen是不能用被动的，注意这里的ed是过去式。
所以可以用的词类就是，accident的主动谓语。原句应为 accident happens. accident takes place. accident be seen.之类的。