"Bir kaza oldu."
Translation:There has been an accident.
That would be "Bir kazaydı." "olmak" basically means "to happen" or "to become." Sometimes, in different tenses and contexts, it can also have the meaning of "there is/was/has been/etc." It isn't the verb to be, but it sometimes has some similarities.
Sometimes we forget to add things. We are human, you know...not to mention volunteers who put a lot of time into this...
I appreciate you helping us throughout this course Alex. Keep up the good work!
It is just wrong. The verb "to happen" cannot take a direct object.