I could buy you could buy he could buy we could buy you could buy they could buy
V-ing form
I could be buying you could be buying he could be buying we could be buying you could be buying they could be buying
Perfect
I could have bought you could have bought he could have bought we could have bought you could have bought they could have bought
V-ing perfect
I could have been buying you could have been buying he could have been buying we could have been buying you could have been buying they could have been buying
Negation
Simple form
I could not buy you could not buy he could not buy we could not buy you could not buy they could not buy
V-ing form
I could not be buying you could not be buying he could not be buying we could not be buying you could not be buying they could not be buying
Perfect
I could not have bought you could not have bought he could not have bought we could not have bought you could not have bought they could not have bought
V-ing perfect
I could not have been buying you could not have been buying he could not have been buying we could not have been buying you could not have been buying they could not have been buying
Interrogative form
Simple form
could I buy? could you buy? could he buy? could we buy? could you buy? could they buy?
V-ing form
could I be buying? could you be buying? could he be buying? could we be buying? could you be buying? could they be buying?
Perfect
could I have bought? could you have bought? could he have bought? could we have bought? could you have bought? could they have bought?
V-ing perfect
could I have been buying? could you have been buying? could he have been buying? could we have been buying? could you have been buying? could they have been buying?
Interro-negative form
Simple form
could I not buy? could you not buy? could he not buy? could we not buy? could you not buy? could they not buy?
V-ing form
could I not be buying? could you not be buying? could he not be buying? could we not be buying? could you not be buying? could they not be buying?
Perfect
could I not have bought? could you not have bought? could he not have bought? could we not have bought? could you not have bought? could they not have bought?
V-ing perfect
could I not have been buying? could you not have been buying? could he not have been buying? could we not have been buying? could you not have been buying? could they not have been buying?