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"have happened" is correct and usable in written English, and it is in the past perfect tense.
You can use it when you want to refer to an event that happened before another event in the past. For example: "By the time the police arrived, the robbery had already happened."
Exact(60)
It shouldn't have happened.
That shouldn't have happened.
What might have happened?
Weirder things have happened.
Could this have happened?
Stranger miracles have happened.
Could that have happened?
Stranger things have happened.
Pandemics have happened before.
Crazier things have happened.
"Worse things have happened".
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com