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