can happen
Regular verb: happen - happened- happened

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
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it can happen
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it can be happening
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it can have happened
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it can have been happening
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Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
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it can't happen
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it can't be happening
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it can't have happened
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it can't have been happening
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Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
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can it happen?
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can it be happening?
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can it have happened?
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can it have been happening?
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Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
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can't it happen?
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can't it be happening?
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can't it have happened?
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can't it have been happening?
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