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