I could box you could box he could box we could box you could box they could box
V-ing form
I could be boxing you could be boxing he could be boxing we could be boxing you could be boxing they could be boxing
Perfect
I could have boxed you could have boxed he could have boxed we could have boxed you could have boxed they could have boxed
V-ing perfect
I could have been boxing you could have been boxing he could have been boxing we could have been boxing you could have been boxing they could have been boxing
Negation
Simple form
I couldn't box you couldn't box he couldn't box we couldn't box you couldn't box they couldn't box
V-ing form
I couldn't be boxing you couldn't be boxing he couldn't be boxing we couldn't be boxing you couldn't be boxing they couldn't be boxing
Perfect
I couldn't have boxed you couldn't have boxed he couldn't have boxed we couldn't have boxed you couldn't have boxed they couldn't have boxed
V-ing perfect
I couldn't have been boxing you couldn't have been boxing he couldn't have been boxing we couldn't have been boxing you couldn't have been boxing they couldn't have been boxing
Interrogative form
Simple form
could I box? could you box? could he box? could we box? could you box? could they box?
V-ing form
could I be boxing? could you be boxing? could he be boxing? could we be boxing? could you be boxing? could they be boxing?
Perfect
could I have boxed? could you have boxed? could he have boxed? could we have boxed? could you have boxed? could they have boxed?
V-ing perfect
could I have been boxing? could you have been boxing? could he have been boxing? could we have been boxing? could you have been boxing? could they have been boxing?
Interro-negative form
Simple form
couldn't I box? couldn't you box? couldn't he box? couldn't we box? couldn't you box? couldn't they box?
V-ing form
couldn't I be boxing? couldn't you be boxing? couldn't he be boxing? couldn't we be boxing? couldn't you be boxing? couldn't they be boxing?
Perfect
couldn't I have boxed? couldn't you have boxed? couldn't he have boxed? couldn't we have boxed? couldn't you have boxed? couldn't they have boxed?
V-ing perfect
couldn't I have been boxing? couldn't you have been boxing? couldn't he have been boxing? couldn't we have been boxing? couldn't you have been boxing? couldn't they have been boxing?