I could improve you could improve she could improve we could improve you could improve they could improve
V-ing form
I could be improving you could be improving she could be improving we could be improving you could be improving they could be improving
Perfect
I could have improved you could have improved she could have improved we could have improved you could have improved they could have improved
V-ing perfect
I could have been improving you could have been improving she could have been improving we could have been improving you could have been improving they could have been improving
Negation
Simple form
I couldn't improve you couldn't improve she couldn't improve we couldn't improve you couldn't improve they couldn't improve
V-ing form
I couldn't be improving you couldn't be improving she couldn't be improving we couldn't be improving you couldn't be improving they couldn't be improving
Perfect
I couldn't have improved you couldn't have improved she couldn't have improved we couldn't have improved you couldn't have improved they couldn't have improved
V-ing perfect
I couldn't have been improving you couldn't have been improving she couldn't have been improving we couldn't have been improving you couldn't have been improving they couldn't have been improving
Interrogative form
Simple form
could I improve? could you improve? could she improve? could we improve? could you improve? could they improve?
V-ing form
could I be improving? could you be improving? could she be improving? could we be improving? could you be improving? could they be improving?
Perfect
could I have improved? could you have improved? could she have improved? could we have improved? could you have improved? could they have improved?
V-ing perfect
could I have been improving? could you have been improving? could she have been improving? could we have been improving? could you have been improving? could they have been improving?
Interro-negative form
Simple form
couldn't I improve? couldn't you improve? couldn't she improve? couldn't we improve? couldn't you improve? couldn't they improve?
V-ing form
couldn't I be improving? couldn't you be improving? couldn't she be improving? couldn't we be improving? couldn't you be improving? couldn't they be improving?
Perfect
couldn't I have improved? couldn't you have improved? couldn't she have improved? couldn't we have improved? couldn't you have improved? couldn't they have improved?
V-ing perfect
couldn't I have been improving? couldn't you have been improving? couldn't she have been improving? couldn't we have been improving? couldn't you have been improving? couldn't they have been improving?