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