English verb conjugation COULD HAPPEN
Regular verb: happen - happened - happened
Affirmation
Simple form
--
it could happen
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V-ing form
--
it could be happening
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Perfect
--
it could have happened
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V-ing perfect
--
it could have been happening
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Negation
Simple form
--
it could not happen
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V-ing form
--
it could not be happening
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Perfect
--
it could not have happened
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V-ing perfect
--
it could not have been happening
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Interrogative form
Simple form
--
could it happen?
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V-ing form
--
could it be happening?
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Perfect
--
could it have happened?
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V-ing perfect
--
could it have been happening?
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Interro-negative form
Simple form
--
could it not happen?
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V-ing form
--
could it not be happening?
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Perfect
--
could it not have happened?
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V-ing perfect
--
could it not have been happening?
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