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 couldn't thumbtack
you couldn't thumbtack
she couldn't thumbtack
we couldn't thumbtack
you couldn't thumbtack
they couldn't thumbtack 	I couldn't be thumbtacking
you couldn't be thumbtacking
she couldn't be thumbtacking
we couldn't be thumbtacking
you couldn't be thumbtacking
they couldn't be thumbtacking	I couldn't have thumbtacked
you couldn't have thumbtacked
she couldn't have thumbtacked
we couldn't have thumbtacked
you couldn't have thumbtacked
they couldn't have thumbtacked	I couldn't have been thumbtacking
you couldn't have been thumbtacking
she couldn't have been thumbtacking
we couldn't have been thumbtacking
you couldn't have been thumbtacking
they couldn't 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 	
couldn't I thumbtack?
couldn't you thumbtack?
couldn't she thumbtack?
couldn't we thumbtack?
couldn't you thumbtack?
couldn't they thumbtack? 	couldn't I be thumbtacking?
couldn't you be thumbtacking?
couldn't she be thumbtacking?
couldn't we be thumbtacking?
couldn't you be thumbtacking?
couldn't they be thumbtacking? 	couldn't I have thumbtacked?
couldn't you have thumbtacked?
couldn't she have thumbtacked?
couldn't we have thumbtacked?
couldn't you have thumbtacked?
couldn't they have thumbtacked? 	couldn't I have been thumbtacking?
couldn't you have been thumbtacking?
couldn't she have been thumbtacking?
couldn't we have been thumbtacking?
couldn't you have been thumbtacking?
couldn't they have been thumbtacking?