happen verb
ADV. 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. | spontaneously Sometimes fun activities just happen spontaneously; at other times they take careful planning. | overnight Change doesn't happen overnight.
VERB + HAPPEN 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.
PREP. to She didn't know what was happening to her.
You can also check other dicts: happen (English, 中文解释 ), wordnet sense, Collins Definition