I could ski you could ski she could ski we could ski you could ski they could ski
V-ing form
I could be skiing you could be skiing she could be skiing we could be skiing you could be skiing they could be skiing
Perfect
I could have skied you could have skied she 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 she 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 she 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 she 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 she 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 she 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 she ski? could we ski? could you ski? could they ski?
V-ing form
could I be skiing? could you be skiing? could she be skiing? could we be skiing? could you be skiing? could they be skiing?
Perfect
could I have skied? could you have skied? could she 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 she 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 she 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 she 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 she 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 she have been skiing? couldn't we have been skiing? couldn't you have been skiing? couldn't they have been skiing?