could yack
Regular verb: yack - yacked- yacked

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could yack
you could yack
he could yack
we could yack
you could yack
they could yack 	I could be yacking
you could be yacking
he could be yacking
we could be yacking
you could be yacking
they could be yacking	I could have yacked
you could have yacked
he could have yacked
we could have yacked
you could have yacked
they could have yacked	I could have been yacking
you could have been yacking
he could have been yacking
we could have been yacking
you could have been yacking
they could have been yacking	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I couldn't yack
you couldn't yack
he couldn't yack
we couldn't yack
you couldn't yack
they couldn't yack 	I couldn't be yacking
you couldn't be yacking
he couldn't be yacking
we couldn't be yacking
you couldn't be yacking
they couldn't be yacking	I couldn't have yacked
you couldn't have yacked
he couldn't have yacked
we couldn't have yacked
you couldn't have yacked
they couldn't have yacked	I couldn't have been yacking
you couldn't have been yacking
he couldn't have been yacking
we couldn't have been yacking
you couldn't have been yacking
they couldn't have been yacking	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I yack?
could you yack?
could he yack?
could we yack?
could you yack?
could they yack? 	could I be yacking?
could you be yacking?
could he be yacking?
could we be yacking?
could you be yacking?
could they be yacking? 	could I have yacked?
could you have yacked?
could he have yacked?
could we have yacked?
could you have yacked?
could they have yacked? 	could I have been yacking?
could you have been yacking?
could he have been yacking?
could we have been yacking?
could you have been yacking?
could they have been yacking? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
couldn't I yack?
couldn't you yack?
couldn't he yack?
couldn't we yack?
couldn't you yack?
couldn't they yack? 	couldn't I be yacking?
couldn't you be yacking?
couldn't he be yacking?
couldn't we be yacking?
couldn't you be yacking?
couldn't they be yacking? 	couldn't I have yacked?
couldn't you have yacked?
couldn't he have yacked?
couldn't we have yacked?
couldn't you have yacked?
couldn't they have yacked? 	couldn't I have been yacking?
couldn't you have been yacking?
couldn't he have been yacking?
couldn't we have been yacking?
couldn't you have been yacking?
couldn't they have been yacking?