Definition of happen verb from the Oxford Advanced American Dictionary

      

    happen

     verb
    verb
    NAmE//ˈhæpən//
     
    Verb Forms present simple I / you / we / they happen
     
    he / she / it happens
     
    past simple happened
     
    -ing form happening
     
     
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  1. 1[intransitive] to take place, especially without being planned You'll never guess what's happened! Accidents like this happen all the time. Let's see what happens next week. I'll be there whatever happens. Is this really happening or is it a dream? I don't know how this happened.
  2. 2 [intransitive] to take place as the result of something She pressed the button but nothing happened. What happens if no one comes to the party? Just plug it in and see what happens.
  3. 3 [transitive] to do or be something by chance happen to be/do something She happened to be out when we called. You don't happen to know his name, do you? This happens to be my favorite restaurant. The door happened to be unlocked. it happens that… It happened that she was out when we called.
  4. 4 [transitive] happen to be something used to tell someone something, especially when you are disagreeing with them or annoyed by what they have said That happens to be my mother you're talking about!
  5. Idioms
    an accident/a disaster waiting to happen
     
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    a thing or person that is very likely to cause danger or a problem in the future because of the condition it is in or the way they behave
    accidents (will) happen
     
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    people say accidents (will) happen to tell someone who has had an accident, for example breaking something, that it does not matter and they should not worry
    anything can/might happen
     
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    used to say that it is not possible to know what the result of something will be
    as it happens/happened
     
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    used when you say something that is surprising, or something connected with what someone else has just said I agree with you, as it happens. As it happens, I have a spare set of keys in my office.
    in the event of something,in the event that something happens
     
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     if something happens In the event of an accident, call this number. Sheila will inherit everything in the event of his death.
    it (just) so happens that…
     
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    by chance It just so happened they'd been invited too.
    these things happen
     
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    used to tell someone not to worry about something they have done “Sorry—I've spilled some wine.” “Never mind. These things happen.”
    Phrasal Verbshappen on somethinghappen to somebody/something
See the Oxford Advanced Learner's Dictionary entry: happen