I could improve you could improve he could improve we could improve you could improve they could improve
V-ing form
I could be improving you could be improving he could be improving we could be improving you could be improving they could be improving
Perfect
I could have improved you could have improved he 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 he could have been improving we could have been improving you could have been improving they could have been improving
Negation
Simple form
I could not improve you could not improve he could not improve we could not improve you could not improve they could not improve
V-ing form
I could not be improving you could not be improving he could not be improving we could not be improving you could not be improving they could not be improving
Perfect
I could not have improved you could not have improved he could not have improved we could not have improved you could not have improved they could not have improved
V-ing perfect
I could not have been improving you could not have been improving he could not have been improving we could not have been improving you could not have been improving they could not have been improving
Interrogative form
Simple form
could I improve? could you improve? could he improve? could we improve? could you improve? could they improve?
V-ing form
could I be improving? could you be improving? could he be improving? could we be improving? could you be improving? could they be improving?
Perfect
could I have improved? could you have improved? could he 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 he have been improving? could we have been improving? could you have been improving? could they have been improving?
Interro-negative form
Simple form
could I not improve? could you not improve? could he not improve? could we not improve? could you not improve? could they not improve?
V-ing form
could I not be improving? could you not be improving? could he not be improving? could we not be improving? could you not be improving? could they not be improving?
Perfect
could I not have improved? could you not have improved? could he not have improved? could we not have improved? could you not have improved? could they not have improved?
V-ing perfect
could I not have been improving? could you not have been improving? could he not have been improving? could we not have been improving? could you not have been improving? could they not have been improving?