I can buy you can buy he can buy we can buy you can buy they can buy
V-ing form
I can be buying you can be buying he can be buying we can be buying you can be buying they can be buying
Perfect
I can have bought you can have bought he can have bought we can have bought you can have bought they can have bought
V-ing perfect
I can have been buying you can have been buying he can have been buying we can have been buying you can have been buying they can have been buying
Negation
Simple form
I can't buy you can't buy he can't buy we can't buy you can't buy they can't buy
V-ing form
I can't be buying you can't be buying he can't be buying we can't be buying you can't be buying they can't be buying
Perfect
I can't have bought you can't have bought he can't have bought we can't have bought you can't have bought they can't have bought
V-ing perfect
I can't have been buying you can't have been buying he can't have been buying we can't have been buying you can't have been buying they can't have been buying
Interrogative form
Simple form
can I buy? can you buy? can he buy? can we buy? can you buy? can they buy?
V-ing form
can I be buying? can you be buying? can he be buying? can we be buying? can you be buying? can they be buying?
Perfect
can I have bought? can you have bought? can he have bought? can we have bought? can you have bought? can they have bought?
V-ing perfect
can I have been buying? can you have been buying? can he have been buying? can we have been buying? can you have been buying? can they have been buying?
Interro-negative form
Simple form
can't I buy? can't you buy? can't he buy? can't we buy? can't you buy? can't they buy?
V-ing form
can't I be buying? can't you be buying? can't he be buying? can't we be buying? can't you be buying? can't they be buying?
Perfect
can't I have bought? can't you have bought? can't he have bought? can't we have bought? can't you have bought? can't they have bought?
V-ing perfect
can't I have been buying? can't you have been buying? can't he have been buying? can't we have been buying? can't you have been buying? can't they have been buying?