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 couldn't fail fail you couldn't fail fail she couldn't fail fail we couldn't fail fail you couldn't fail fail they couldn't fail fail
V-ing form
I couldn't be fail failing you couldn't be fail failing she couldn't be fail failing we couldn't be fail failing you couldn't be fail failing they couldn't be fail failing
Perfect
I couldn't have fail failed you couldn't have fail failed she couldn't have fail failed we couldn't have fail failed you couldn't have fail failed they couldn't have fail failed
V-ing perfect
I couldn't have been fail failing you couldn't have been fail failing she couldn't have been fail failing we couldn't have been fail failing you couldn't have been fail failing they couldn't 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
couldn't I fail fail? couldn't you fail fail? couldn't she fail fail? couldn't we fail fail? couldn't you fail fail? couldn't they fail fail?
V-ing form
couldn't I be fail failing? couldn't you be fail failing? couldn't she be fail failing? couldn't we be fail failing? couldn't you be fail failing? couldn't they be fail failing?
Perfect
couldn't I have fail failed? couldn't you have fail failed? couldn't she have fail failed? couldn't we have fail failed? couldn't you have fail failed? couldn't they have fail failed?
V-ing perfect
couldn't I have been fail failing? couldn't you have been fail failing? couldn't she have been fail failing? couldn't we have been fail failing? couldn't you have been fail failing? couldn't they have been fail failing?