I could bill you could bill she could bill we could bill you could bill they could bill
V-ing form
I could be billing you could be billing she could be billing we could be billing you could be billing they could be billing
Perfect
I could have billed you could have billed she could have billed we could have billed you could have billed they could have billed
V-ing perfect
I could have been billing you could have been billing she could have been billing we could have been billing you could have been billing they could have been billing
Negation
Simple form
I could not bill you could not bill she could not bill we could not bill you could not bill they could not bill
V-ing form
I could not be billing you could not be billing she could not be billing we could not be billing you could not be billing they could not be billing
Perfect
I could not have billed you could not have billed she could not have billed we could not have billed you could not have billed they could not have billed
V-ing perfect
I could not have been billing you could not have been billing she could not have been billing we could not have been billing you could not have been billing they could not have been billing
Interrogative form
Simple form
could I bill? could you bill? could she bill? could we bill? could you bill? could they bill?
V-ing form
could I be billing? could you be billing? could she be billing? could we be billing? could you be billing? could they be billing?
Perfect
could I have billed? could you have billed? could she have billed? could we have billed? could you have billed? could they have billed?
V-ing perfect
could I have been billing? could you have been billing? could she have been billing? could we have been billing? could you have been billing? could they have been billing?
Interro-negative form
Simple form
could I not bill? could you not bill? could she not bill? could we not bill? could you not bill? could they not bill?
V-ing form
could I not be billing? could you not be billing? could she not be billing? could we not be billing? could you not be billing? could they not be billing?
Perfect
could I not have billed? could you not have billed? could she not have billed? could we not have billed? could you not have billed? could they not have billed?
V-ing perfect
could I not have been billing? could you not have been billing? could she not have been billing? could we not have been billing? could you not have been billing? could they not have been billing?