I could butter you could butter he could butter we could butter you could butter they could butter
V-ing form
I could be buttering you could be buttering he could be buttering we could be buttering you could be buttering they could be buttering
Perfect
I could have buttered you could have buttered he could have buttered we could have buttered you could have buttered they could have buttered
V-ing perfect
I could have been buttering you could have been buttering he could have been buttering we could have been buttering you could have been buttering they could have been buttering
Negation
Simple form
I could not butter you could not butter he could not butter we could not butter you could not butter they could not butter
V-ing form
I could not be buttering you could not be buttering he could not be buttering we could not be buttering you could not be buttering they could not be buttering
Perfect
I could not have buttered you could not have buttered he could not have buttered we could not have buttered you could not have buttered they could not have buttered
V-ing perfect
I could not have been buttering you could not have been buttering he could not have been buttering we could not have been buttering you could not have been buttering they could not have been buttering
Interrogative form
Simple form
could I butter? could you butter? could he butter? could we butter? could you butter? could they butter?
V-ing form
could I be buttering? could you be buttering? could he be buttering? could we be buttering? could you be buttering? could they be buttering?
Perfect
could I have buttered? could you have buttered? could he have buttered? could we have buttered? could you have buttered? could they have buttered?
V-ing perfect
could I have been buttering? could you have been buttering? could he have been buttering? could we have been buttering? could you have been buttering? could they have been buttering?
Interro-negative form
Simple form
could I not butter? could you not butter? could he not butter? could we not butter? could you not butter? could they not butter?
V-ing form
could I not be buttering? could you not be buttering? could he not be buttering? could we not be buttering? could you not be buttering? could they not be buttering?
Perfect
could I not have buttered? could you not have buttered? could he not have buttered? could we not have buttered? could you not have buttered? could they not have buttered?
V-ing perfect
could I not have been buttering? could you not have been buttering? could he not have been buttering? could we not have been buttering? could you not have been buttering? could they not have been buttering?