could happen
Regular verb: happen - happened- happened

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