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