could jackknife
Regular verb: jackknife - jackknifed- jackknifed

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could jackknife
you could jackknife
she could jackknife
we could jackknife
you could jackknife
they could jackknife 	I could be jackknifing
you could be jackknifing
she could be jackknifing
we could be jackknifing
you could be jackknifing
they could be jackknifing	I could have jackknifed
you could have jackknifed
she could have jackknifed
we could have jackknifed
you could have jackknifed
they could have jackknifed	I could have been jackknifing
you could have been jackknifing
she could have been jackknifing
we could have been jackknifing
you could have been jackknifing
they could have been jackknifing	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could not jackknife
you could not jackknife
she could not jackknife
we could not jackknife
you could not jackknife
they could not jackknife 	I could not be jackknifing
you could not be jackknifing
she could not be jackknifing
we could not be jackknifing
you could not be jackknifing
they could not be jackknifing	I could not have jackknifed
you could not have jackknifed
she could not have jackknifed
we could not have jackknifed
you could not have jackknifed
they could not have jackknifed	I could not have been jackknifing
you could not have been jackknifing
she could not have been jackknifing
we could not have been jackknifing
you could not have been jackknifing
they could not have been jackknifing	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I jackknife?
could you jackknife?
could she jackknife?
could we jackknife?
could you jackknife?
could they jackknife? 	could I be jackknifing?
could you be jackknifing?
could she be jackknifing?
could we be jackknifing?
could you be jackknifing?
could they be jackknifing? 	could I have jackknifed?
could you have jackknifed?
could she have jackknifed?
could we have jackknifed?
could you have jackknifed?
could they have jackknifed? 	could I have been jackknifing?
could you have been jackknifing?
could she have been jackknifing?
could we have been jackknifing?
could you have been jackknifing?
could they have been jackknifing? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I not jackknife?
could you not jackknife?
could she not jackknife?
could we not jackknife?
could you not jackknife?
could they not jackknife? 	could I not be jackknifing?
could you not be jackknifing?
could she not be jackknifing?
could we not be jackknifing?
could you not be jackknifing?
could they not be jackknifing? 	could I not have jackknifed?
could you not have jackknifed?
could she not have jackknifed?
could we not have jackknifed?
could you not have jackknifed?
could they not have jackknifed? 	could I not have been jackknifing?
could you not have been jackknifing?
could she not have been jackknifing?
could we not have been jackknifing?
could you not have been jackknifing?
could they not have been jackknifing?