I can ski you can ski he can ski we can ski you can ski they can ski
V-ing form
I can be skiing you can be skiing he can be skiing we can be skiing you can be skiing they can be skiing
Perfect
I can have skied you can have skied he can have skied we can have skied you can have skied they can have skied
V-ing perfect
I can have been skiing you can have been skiing he can have been skiing we can have been skiing you can have been skiing they can have been skiing
Negation
Simple form
I cannot ski you cannot ski he cannot ski we cannot ski you cannot ski they cannot ski
V-ing form
I cannot be skiing you cannot be skiing he cannot be skiing we cannot be skiing you cannot be skiing they cannot be skiing
Perfect
I cannot have skied you cannot have skied he cannot have skied we cannot have skied you cannot have skied they cannot have skied
V-ing perfect
I cannot have been skiing you cannot have been skiing he cannot have been skiing we cannot have been skiing you cannot have been skiing they cannot have been skiing
Interrogative form
Simple form
can I ski? can you ski? can he ski? can we ski? can you ski? can they ski?
V-ing form
can I be skiing? can you be skiing? can he be skiing? can we be skiing? can you be skiing? can they be skiing?
Perfect
can I have skied? can you have skied? can he have skied? can we have skied? can you have skied? can they have skied?
V-ing perfect
can I have been skiing? can you have been skiing? can he have been skiing? can we have been skiing? can you have been skiing? can they have been skiing?
Interro-negative form
Simple form
can I not ski? can you not ski? can he not ski? can we not ski? can you not ski? can they not ski?
V-ing form
can I not be skiing? can you not be skiing? can he not be skiing? can we not be skiing? can you not be skiing? can they not be skiing?
Perfect
can I not have skied? can you not have skied? can he not have skied? can we not have skied? can you not have skied? can they not have skied?
V-ing perfect
can I not have been skiing? can you not have been skiing? can he not have been skiing? can we not have been skiing? can you not have been skiing? can they not have been skiing?