could knock
Regular verb: knock - knocked- knocked

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could knock
you could knock
she could knock
we could knock
you could knock
they could knock 	I could be knocking
you could be knocking
she could be knocking
we could be knocking
you could be knocking
they could be knocking	I could have knocked
you could have knocked
she could have knocked
we could have knocked
you could have knocked
they could have knocked	I could have been knocking
you could have been knocking
she could have been knocking
we could have been knocking
you could have been knocking
they could have been knocking	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I couldn't knock
you couldn't knock
she couldn't knock
we couldn't knock
you couldn't knock
they couldn't knock 	I couldn't be knocking
you couldn't be knocking
she couldn't be knocking
we couldn't be knocking
you couldn't be knocking
they couldn't be knocking	I couldn't have knocked
you couldn't have knocked
she couldn't have knocked
we couldn't have knocked
you couldn't have knocked
they couldn't have knocked	I couldn't have been knocking
you couldn't have been knocking
she couldn't have been knocking
we couldn't have been knocking
you couldn't have been knocking
they couldn't have been knocking	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I knock?
could you knock?
could she knock?
could we knock?
could you knock?
could they knock? 	could I be knocking?
could you be knocking?
could she be knocking?
could we be knocking?
could you be knocking?
could they be knocking? 	could I have knocked?
could you have knocked?
could she have knocked?
could we have knocked?
could you have knocked?
could they have knocked? 	could I have been knocking?
could you have been knocking?
could she have been knocking?
could we have been knocking?
could you have been knocking?
could they have been knocking? 	

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