I could walk you could walk she could walk we could walk you could walk they could walk
V-ing form
I could be walking you could be walking she could be walking we could be walking you could be walking they could be walking
Perfect
I could have walked you could have walked she 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 she could have been walking we could have been walking you could have been walking they could have been walking
Negation
Simple form
I could not walk you could not walk she could not walk we could not walk you could not walk they could not walk
V-ing form
I could not be walking you could not be walking she could not be walking we could not be walking you could not be walking they could not be walking
Perfect
I could not have walked you could not have walked she could not have walked we could not have walked you could not have walked they could not have walked
V-ing perfect
I could not have been walking you could not have been walking she could not have been walking we could not have been walking you could not have been walking they could not have been walking
Interrogative form
Simple form
could I walk? could you walk? could she walk? could we walk? could you walk? could they walk?
V-ing form
could I be walking? could you be walking? could she be walking? could we be walking? could you be walking? could they be walking?
Perfect
could I have walked? could you have walked? could she 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 she have been walking? could we have been walking? could you have been walking? could they have been walking?
Interro-negative form
Simple form
could I not walk? could you not walk? could she not walk? could we not walk? could you not walk? could they not walk?
V-ing form
could I not be walking? could you not be walking? could she not be walking? could we not be walking? could you not be walking? could they not be walking?
Perfect
could I not have walked? could you not have walked? could she not have walked? could we not have walked? could you not have walked? could they not have walked?
V-ing perfect
could I not have been walking? could you not have been walking? could she not have been walking? could we not have been walking? could you not have been walking? could they not have been walking?