I could skate you could skate he could skate we could skate you could skate they could skate
V-ing form
I could be skating you could be skating he could be skating we could be skating you could be skating they could be skating
Perfect
I could have skated you could have skated he could have skated we could have skated you could have skated they could have skated
V-ing perfect
I could have been skating you could have been skating he could have been skating we could have been skating you could have been skating they could have been skating
Negation
Simple form
I could not skate you could not skate he could not skate we could not skate you could not skate they could not skate
V-ing form
I could not be skating you could not be skating he could not be skating we could not be skating you could not be skating they could not be skating
Perfect
I could not have skated you could not have skated he could not have skated we could not have skated you could not have skated they could not have skated
V-ing perfect
I could not have been skating you could not have been skating he could not have been skating we could not have been skating you could not have been skating they could not have been skating
Interrogative form
Simple form
could I skate? could you skate? could he skate? could we skate? could you skate? could they skate?
V-ing form
could I be skating? could you be skating? could he be skating? could we be skating? could you be skating? could they be skating?
Perfect
could I have skated? could you have skated? could he have skated? could we have skated? could you have skated? could they have skated?
V-ing perfect
could I have been skating? could you have been skating? could he have been skating? could we have been skating? could you have been skating? could they have been skating?
Interro-negative form
Simple form
could I not skate? could you not skate? could he not skate? could we not skate? could you not skate? could they not skate?
V-ing form
could I not be skating? could you not be skating? could he not be skating? could we not be skating? could you not be skating? could they not be skating?
Perfect
could I not have skated? could you not have skated? could he not have skated? could we not have skated? could you not have skated? could they not have skated?
V-ing perfect
could I not have been skating? could you not have been skating? could he not have been skating? could we not have been skating? could you not have been skating? could they not have been skating?