Warning, this verb may not exist. Conjugation is given as example.
Affirmation
Simple form
I could fail fail you could fail fail she could fail fail we could fail fail you could fail fail they could fail fail
V-ing form
I could be fail failing you could be fail failing she could be fail failing we could be fail failing you could be fail failing they could be fail failing
Perfect
I could have fail failed you could have fail failed she could have fail failed we could have fail failed you could have fail failed they could have fail failed
V-ing perfect
I could have been fail failing you could have been fail failing she could have been fail failing we could have been fail failing you could have been fail failing they could have been fail failing
Negation
Simple form
I could not fail fail you could not fail fail she could not fail fail we could not fail fail you could not fail fail they could not fail fail
V-ing form
I could not be fail failing you could not be fail failing she could not be fail failing we could not be fail failing you could not be fail failing they could not be fail failing
Perfect
I could not have fail failed you could not have fail failed she could not have fail failed we could not have fail failed you could not have fail failed they could not have fail failed
V-ing perfect
I could not have been fail failing you could not have been fail failing she could not have been fail failing we could not have been fail failing you could not have been fail failing they could not have been fail failing
Interrogative form
Simple form
could I fail fail? could you fail fail? could she fail fail? could we fail fail? could you fail fail? could they fail fail?
V-ing form
could I be fail failing? could you be fail failing? could she be fail failing? could we be fail failing? could you be fail failing? could they be fail failing?
Perfect
could I have fail failed? could you have fail failed? could she have fail failed? could we have fail failed? could you have fail failed? could they have fail failed?
V-ing perfect
could I have been fail failing? could you have been fail failing? could she have been fail failing? could we have been fail failing? could you have been fail failing? could they have been fail failing?
Interro-negative form
Simple form
could I not fail fail? could you not fail fail? could she not fail fail? could we not fail fail? could you not fail fail? could they not fail fail?
V-ing form
could I not be fail failing? could you not be fail failing? could she not be fail failing? could we not be fail failing? could you not be fail failing? could they not be fail failing?
Perfect
could I not have fail failed? could you not have fail failed? could she not have fail failed? could we not have fail failed? could you not have fail failed? could they not have fail failed?
V-ing perfect
could I not have been fail failing? could you not have been fail failing? could she not have been fail failing? could we not have been fail failing? could you not have been fail failing? could they not have been fail failing?