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