I could back you could back he could back we could back you could back they could back
V-ing form
I could be backing you could be backing he could be backing we could be backing you could be backing they could be backing
Perfect
I could have backed you could have backed he could have backed we could have backed you could have backed they could have backed
V-ing perfect
I could have been backing you could have been backing he could have been backing we could have been backing you could have been backing they could have been backing
Negation
Simple form
I could not back you could not back he could not back we could not back you could not back they could not back
V-ing form
I could not be backing you could not be backing he could not be backing we could not be backing you could not be backing they could not be backing
Perfect
I could not have backed you could not have backed he could not have backed we could not have backed you could not have backed they could not have backed
V-ing perfect
I could not have been backing you could not have been backing he could not have been backing we could not have been backing you could not have been backing they could not have been backing
Interrogative form
Simple form
could I back? could you back? could he back? could we back? could you back? could they back?
V-ing form
could I be backing? could you be backing? could he be backing? could we be backing? could you be backing? could they be backing?
Perfect
could I have backed? could you have backed? could he have backed? could we have backed? could you have backed? could they have backed?
V-ing perfect
could I have been backing? could you have been backing? could he have been backing? could we have been backing? could you have been backing? could they have been backing?
Interro-negative form
Simple form
could I not back? could you not back? could he not back? could we not back? could you not back? could they not back?
V-ing form
could I not be backing? could you not be backing? could he not be backing? could we not be backing? could you not be backing? could they not be backing?
Perfect
could I not have backed? could you not have backed? could he not have backed? could we not have backed? could you not have backed? could they not have backed?
V-ing perfect
could I not have been backing? could you not have been backing? could he not have been backing? could we not have been backing? could you not have been backing? could they not have been backing?