Happen is used after these nouns:
break-in

actually, really
She couldn't quite believe that all this was actually happening to her.

just
I don't remember learning to swim, it just happened.

never
spontaneously
Sometimes fun activities just happen spontaneously; at other times they take careful planning.

overnight, quickly
Change doesn't happen overnight.

again

be going to
They could only wait and see what was going to happen.

be likely to
be bound to
Mistakes are bound to happen sometimes.

tend to
What tends to happen is that students spend the first week of the course in a blind panic, but settle down by the second or third week.

want sth to
make sth
You have to make things happen if you want them to happen.

let sth
Don't just sit back and let it happen.

to
She didn't know what was happening to her.

Happen is used with these nouns as the subject:
abuse, accident, assault, attack, avalanche, breakthrough, burglary, catastrophe, change, coincidence, collision, crash, crime, deal, death, disaster, earthquake, emergency, eruption, event, evolution, explosion, fit, incident, injury, innovation, killing, leak, magic, meeting, merger, miracle, mistake, murder, phenomenon, riot, skirmish, smash, thing, tragedy, transformation