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