I could tree you could tree she could tree we could tree you could tree they could tree
V-ing form
I could be treeing you could be treeing she could be treeing we could be treeing you could be treeing they could be treeing
Perfect
I could have treed you could have treed she could have treed we could have treed you could have treed they could have treed
V-ing perfect
I could have been treeing you could have been treeing she could have been treeing we could have been treeing you could have been treeing they could have been treeing
Negation
Simple form
I could not tree you could not tree she could not tree we could not tree you could not tree they could not tree
V-ing form
I could not be treeing you could not be treeing she could not be treeing we could not be treeing you could not be treeing they could not be treeing
Perfect
I could not have treed you could not have treed she could not have treed we could not have treed you could not have treed they could not have treed
V-ing perfect
I could not have been treeing you could not have been treeing she could not have been treeing we could not have been treeing you could not have been treeing they could not have been treeing
Interrogative form
Simple form
could I tree? could you tree? could she tree? could we tree? could you tree? could they tree?
V-ing form
could I be treeing? could you be treeing? could she be treeing? could we be treeing? could you be treeing? could they be treeing?
Perfect
could I have treed? could you have treed? could she have treed? could we have treed? could you have treed? could they have treed?
V-ing perfect
could I have been treeing? could you have been treeing? could she have been treeing? could we have been treeing? could you have been treeing? could they have been treeing?
Interro-negative form
Simple form
could I not tree? could you not tree? could she not tree? could we not tree? could you not tree? could they not tree?
V-ing form
could I not be treeing? could you not be treeing? could she not be treeing? could we not be treeing? could you not be treeing? could they not be treeing?
Perfect
could I not have treed? could you not have treed? could she not have treed? could we not have treed? could you not have treed? could they not have treed?
V-ing perfect
could I not have been treeing? could you not have been treeing? could she not have been treeing? could we not have been treeing? could you not have been treeing? could they not have been treeing?