could thumbtack
Regular verb: thumbtack - thumbtacked- thumbtacked

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could thumbtack
you could thumbtack
she could thumbtack
we could thumbtack
you could thumbtack
they could thumbtack 	I could be thumbtacking
you could be thumbtacking
she could be thumbtacking
we could be thumbtacking
you could be thumbtacking
they could be thumbtacking	I could have thumbtacked
you could have thumbtacked
she could have thumbtacked
we could have thumbtacked
you could have thumbtacked
they could have thumbtacked	I could have been thumbtacking
you could have been thumbtacking
she could have been thumbtacking
we could have been thumbtacking
you could have been thumbtacking
they could have been thumbtacking	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could not thumbtack
you could not thumbtack
she could not thumbtack
we could not thumbtack
you could not thumbtack
they could not thumbtack 	I could not be thumbtacking
you could not be thumbtacking
she could not be thumbtacking
we could not be thumbtacking
you could not be thumbtacking
they could not be thumbtacking	I could not have thumbtacked
you could not have thumbtacked
she could not have thumbtacked
we could not have thumbtacked
you could not have thumbtacked
they could not have thumbtacked	I could not have been thumbtacking
you could not have been thumbtacking
she could not have been thumbtacking
we could not have been thumbtacking
you could not have been thumbtacking
they could not have been thumbtacking	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I thumbtack?
could you thumbtack?
could she thumbtack?
could we thumbtack?
could you thumbtack?
could they thumbtack? 	could I be thumbtacking?
could you be thumbtacking?
could she be thumbtacking?
could we be thumbtacking?
could you be thumbtacking?
could they be thumbtacking? 	could I have thumbtacked?
could you have thumbtacked?
could she have thumbtacked?
could we have thumbtacked?
could you have thumbtacked?
could they have thumbtacked? 	could I have been thumbtacking?
could you have been thumbtacking?
could she have been thumbtacking?
could we have been thumbtacking?
could you have been thumbtacking?
could they have been thumbtacking? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I not thumbtack?
could you not thumbtack?
could she not thumbtack?
could we not thumbtack?
could you not thumbtack?
could they not thumbtack? 	could I not be thumbtacking?
could you not be thumbtacking?
could she not be thumbtacking?
could we not be thumbtacking?
could you not be thumbtacking?
could they not be thumbtacking? 	could I not have thumbtacked?
could you not have thumbtacked?
could she not have thumbtacked?
could we not have thumbtacked?
could you not have thumbtacked?
could they not have thumbtacked? 	could I not have been thumbtacking?
could you not have been thumbtacking?
could she not have been thumbtacking?
could we not have been thumbtacking?
could you not have been thumbtacking?
could they not have been thumbtacking?