could stack
Regular verb: stack - stacked- stacked

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could stack
you could stack
she could stack
we could stack
you could stack
they could stack 	I could be stacking
you could be stacking
she could be stacking
we could be stacking
you could be stacking
they could be stacking	I could have stacked
you could have stacked
she could have stacked
we could have stacked
you could have stacked
they could have stacked	I could have been stacking
you could have been stacking
she could have been stacking
we could have been stacking
you could have been stacking
they could have been stacking	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could not stack
you could not stack
she could not stack
we could not stack
you could not stack
they could not stack 	I could not be stacking
you could not be stacking
she could not be stacking
we could not be stacking
you could not be stacking
they could not be stacking	I could not have stacked
you could not have stacked
she could not have stacked
we could not have stacked
you could not have stacked
they could not have stacked	I could not have been stacking
you could not have been stacking
she could not have been stacking
we could not have been stacking
you could not have been stacking
they could not have been stacking	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I stack?
could you stack?
could she stack?
could we stack?
could you stack?
could they stack? 	could I be stacking?
could you be stacking?
could she be stacking?
could we be stacking?
could you be stacking?
could they be stacking? 	could I have stacked?
could you have stacked?
could she have stacked?
could we have stacked?
could you have stacked?
could they have stacked? 	could I have been stacking?
could you have been stacking?
could she have been stacking?
could we have been stacking?
could you have been stacking?
could they have been stacking? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I not stack?
could you not stack?
could she not stack?
could we not stack?
could you not stack?
could they not stack? 	could I not be stacking?
could you not be stacking?
could she not be stacking?
could we not be stacking?
could you not be stacking?
could they not be stacking? 	could I not have stacked?
could you not have stacked?
could she not have stacked?
could we not have stacked?
could you not have stacked?
could they not have stacked? 	could I not have been stacking?
could you not have been stacking?
could she not have been stacking?
could we not have been stacking?
could you not have been stacking?
could they not have been stacking?