happen

Here's an accident about to happen.
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[V] 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.
[V] to take place as the result of sth: She pressed the button but nothing happened. ◆ What happens if nobody comes to the party? ◆ Just plug it in and see what happens.
to do or be sth by chance: [V to inf] She happened to be out when we called. ◆ You don't happen to know his name, do you? ◆ This happens to be my favourite restaurant. ◆ The door happened to be unlocked. ◆ [V that] It happened that she was out when we called.
[V to inf] used to tell sb sth, especially when you are disagreeing with them or annoyed by what they have said: That happens to be my mother you're talking about!
Idioms: anything can / might happen used to say that it is not possible to know what the result of sth will be
as it happens / happened used when you say sth that is surprising, or sth connected with what sb else has just said: I agree with you, as it happens. ◆ As it happens, I have a spare set of keys in my office.
it (just) so happens that ... by chance: It just so happened they'd been invited too.
these things happen used to tell sb not to worry about sth they have done: 'Sorry-I've spilt some wine.' 'Never mind. These things happen.'
more at ACCIDENT, EVENT, WAIT v.
Phrasal Verbs: happen on sth (old-fashioned) to find sth by chance
happen to sb/sth to have an effect on sb/sth: I hope nothing (= nothing unpleasant) has happened to them. ◆ It's the best thing that has ever happened to me. ◆ What's happened to your car? ◆ Do you know what happened to Gill Lovecy (= have you any news about her)?

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WHICH WORD?
happen / occur / take place
Happen is the usual word that you use to refer to events that are not planned or expected: You look terrible - what's happened? The word is also used to talk about one event resulting from another:
What happened when you told him the news?
Occur meaning 'happen' is used only in formal situations: Police report that the accident occurred at about 9.30 p.m.
Take place is also quite formal and is usually used to talk about an event that has been planned or arranged or when people take an active part in sth: The festival takes place in July every year. ◆ Filming took place in Ireland. In informal language you would probably say:
The festival is in July every year. ◆ The movie was filmed in Ireland.

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