I can knock you can knock she can knock we can knock you can knock they can knock
V-ing form
I can be knocking you can be knocking she can be knocking we can be knocking you can be knocking they can be knocking
Perfect
I can have knocked you can have knocked she can have knocked we can have knocked you can have knocked they can have knocked
V-ing perfect
I can have been knocking you can have been knocking she can have been knocking we can have been knocking you can have been knocking they can have been knocking
Negation
Simple form
I can't knock you can't knock she can't knock we can't knock you can't knock they can't knock
V-ing form
I can't be knocking you can't be knocking she can't be knocking we can't be knocking you can't be knocking they can't be knocking
Perfect
I can't have knocked you can't have knocked she can't have knocked we can't have knocked you can't have knocked they can't have knocked
V-ing perfect
I can't have been knocking you can't have been knocking she can't have been knocking we can't have been knocking you can't have been knocking they can't have been knocking
Interrogative form
Simple form
can I knock? can you knock? can she knock? can we knock? can you knock? can they knock?
V-ing form
can I be knocking? can you be knocking? can she be knocking? can we be knocking? can you be knocking? can they be knocking?
Perfect
can I have knocked? can you have knocked? can she have knocked? can we have knocked? can you have knocked? can they have knocked?
V-ing perfect
can I have been knocking? can you have been knocking? can she have been knocking? can we have been knocking? can you have been knocking? can they have been knocking?
Interro-negative form
Simple form
can't I knock? can't you knock? can't she knock? can't we knock? can't you knock? can't they knock?
V-ing form
can't I be knocking? can't you be knocking? can't she be knocking? can't we be knocking? can't you be knocking? can't they be knocking?
Perfect
can't I have knocked? can't you have knocked? can't she have knocked? can't we have knocked? can't you have knocked? can't they have knocked?
V-ing perfect
can't I have been knocking? can't you have been knocking? can't she have been knocking? can't we have been knocking? can't you have been knocking? can't they have been knocking?