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