I could err you could err she could err we could err you could err they could err
V-ing form
I could be erring you could be erring she could be erring we could be erring you could be erring they could be erring
Perfect
I could have erred you could have erred she could have erred we could have erred you could have erred they could have erred
V-ing perfect
I could have been erring you could have been erring she could have been erring we could have been erring you could have been erring they could have been erring
Negation
Simple form
I could not err you could not err she could not err we could not err you could not err they could not err
V-ing form
I could not be erring you could not be erring she could not be erring we could not be erring you could not be erring they could not be erring
Perfect
I could not have erred you could not have erred she could not have erred we could not have erred you could not have erred they could not have erred
V-ing perfect
I could not have been erring you could not have been erring she could not have been erring we could not have been erring you could not have been erring they could not have been erring
Interrogative form
Simple form
could I err? could you err? could she err? could we err? could you err? could they err?
V-ing form
could I be erring? could you be erring? could she be erring? could we be erring? could you be erring? could they be erring?
Perfect
could I have erred? could you have erred? could she have erred? could we have erred? could you have erred? could they have erred?
V-ing perfect
could I have been erring? could you have been erring? could she have been erring? could we have been erring? could you have been erring? could they have been erring?
Interro-negative form
Simple form
could I not err? could you not err? could she not err? could we not err? could you not err? could they not err?
V-ing form
could I not be erring? could you not be erring? could she not be erring? could we not be erring? could you not be erring? could they not be erring?
Perfect
could I not have erred? could you not have erred? could she not have erred? could we not have erred? could you not have erred? could they not have erred?
V-ing perfect
could I not have been erring? could you not have been erring? could she not have been erring? could we not have been erring? could you not have been erring? could they not have been erring?