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