Inflections of 'occur' (v): (⇒ conjugate)
v 3rd person singular
v pres pverb, present participle: -ing verb used descriptively or to form progressive verb--for example, "a singing bird," "It is singing."
v pastverb, past simple: Past tense--for example, "He saw the man." "She laughed."
v past pverb, past participle: Verb form used descriptively or to form verbs--for example, "the locked door," "The door has been locked."
WordReference Collins English Usage © 2019
You can say that an event occurs.
The accident occurred at 8:40 a.m.
Mistakes are bound to occur.
However, you only use occur to talk about events which are not planned.
Occur is a fairly formal word. In conversation and in less formal writing, you usually say that an event happens.
You might have noticed what happened on Tuesday.
A curious thing has happened.
➜ See happen
Be careful
Don't say that a planned event ‘occurs’ or ‘happens’. Say that it takes place.
The first meeting of this committee took place on 9 January.
These lessons took place twice a week.
Don't use ‘occur to’ to say that someone is affected by an event. Don't say, for example, ‘I wonder what’s occurred to Jane'. Say ‘I wonder what’s happened to Jane'.
She no longer cared what happened to her.
It couldn't have happened to a nicer man.
'occur' also found in these entries:

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