I could swim you could swim he could swim we could swim you could swim they could swim
V-ing form
I could be swimming you could be swimming he could be swimming we could be swimming you could be swimming they could be swimming
Perfect
I could have swum you could have swum he could have swum we could have swum you could have swum they could have swum
V-ing perfect
I could have been swimming you could have been swimming he could have been swimming we could have been swimming you could have been swimming they could have been swimming
Negation
Simple form
I could not swim you could not swim he could not swim we could not swim you could not swim they could not swim
V-ing form
I could not be swimming you could not be swimming he could not be swimming we could not be swimming you could not be swimming they could not be swimming
Perfect
I could not have swum you could not have swum he could not have swum we could not have swum you could not have swum they could not have swum
V-ing perfect
I could not have been swimming you could not have been swimming he could not have been swimming we could not have been swimming you could not have been swimming they could not have been swimming
Interrogative form
Simple form
could I swim? could you swim? could he swim? could we swim? could you swim? could they swim?
V-ing form
could I be swimming? could you be swimming? could he be swimming? could we be swimming? could you be swimming? could they be swimming?
Perfect
could I have swum? could you have swum? could he have swum? could we have swum? could you have swum? could they have swum?
V-ing perfect
could I have been swimming? could you have been swimming? could he have been swimming? could we have been swimming? could you have been swimming? could they have been swimming?
Interro-negative form
Simple form
could I not swim? could you not swim? could he not swim? could we not swim? could you not swim? could they not swim?
V-ing form
could I not be swimming? could you not be swimming? could he not be swimming? could we not be swimming? could you not be swimming? could they not be swimming?
Perfect
could I not have swum? could you not have swum? could he not have swum? could we not have swum? could you not have swum? could they not have swum?
V-ing perfect
could I not have been swimming? could you not have been swimming? could he not have been swimming? could we not have been swimming? could you not have been swimming? could they not have been swimming?