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