English verb conjugation MUST HAPPEN
Regular verb: happen - happened - happened
Affirmation
Simple form
--
it must happen
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V-ing form
--
it must be happening
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Perfect
--
it must have happened
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V-ing perfect
--
it must have been happening
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Negation
Simple form
--
it mustn't happen
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V-ing form
--
it mustn't be happening
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Perfect
--
it mustn't have happened
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V-ing perfect
--
it mustn't have been happening
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Interrogative form
Simple form
--
must it happen?
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V-ing form
--
must it be happening?
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Perfect
--
must it have happened?
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V-ing perfect
--
must it have been happening?
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Interro-negative form
Simple form
--
mustn't it happen?
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V-ing form
--
mustn't it be happening?
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Perfect
--
mustn't it have happened?
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V-ing perfect
--
mustn't it have been happening?
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