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