I could knock you could knock he could knock we could knock you could knock they could knock
V-ing form
I could be knocking you could be knocking he could be knocking we could be knocking you could be knocking they could be knocking
Perfect
I could have knocked you could have knocked he could have knocked we could have knocked you could have knocked they could have knocked
V-ing perfect
I could have been knocking you could have been knocking he could have been knocking we could have been knocking you could have been knocking they could have been knocking
Negation
Simple form
I could not knock you could not knock he could not knock we could not knock you could not knock they could not knock
V-ing form
I could not be knocking you could not be knocking he could not be knocking we could not be knocking you could not be knocking they could not be knocking
Perfect
I could not have knocked you could not have knocked he could not have knocked we could not have knocked you could not have knocked they could not have knocked
V-ing perfect
I could not have been knocking you could not have been knocking he could not have been knocking we could not have been knocking you could not have been knocking they could not have been knocking
Interrogative form
Simple form
could I knock? could you knock? could he knock? could we knock? could you knock? could they knock?
V-ing form
could I be knocking? could you be knocking? could he be knocking? could we be knocking? could you be knocking? could they be knocking?
Perfect
could I have knocked? could you have knocked? could he have knocked? could we have knocked? could you have knocked? could they have knocked?
V-ing perfect
could I have been knocking? could you have been knocking? could he have been knocking? could we have been knocking? could you have been knocking? could they have been knocking?
Interro-negative form
Simple form
could I not knock? could you not knock? could he not knock? could we not knock? could you not knock? could they not knock?
V-ing form
could I not be knocking? could you not be knocking? could he not be knocking? could we not be knocking? could you not be knocking? could they not be knocking?
Perfect
could I not have knocked? could you not have knocked? could he not have knocked? could we not have knocked? could you not have knocked? could they not have knocked?
V-ing perfect
could I not have been knocking? could you not have been knocking? could he not have been knocking? could we not have been knocking? could you not have been knocking? could they not have been knocking?