I could shorten you could shorten he could shorten we could shorten you could shorten they could shorten
V-ing form
I could be shortening you could be shortening he could be shortening we could be shortening you could be shortening they could be shortening
Perfect
I could have shortened you could have shortened he could have shortened we could have shortened you could have shortened they could have shortened
V-ing perfect
I could have been shortening you could have been shortening he could have been shortening we could have been shortening you could have been shortening they could have been shortening
Negation
Simple form
I couldn't shorten you couldn't shorten he couldn't shorten we couldn't shorten you couldn't shorten they couldn't shorten
V-ing form
I couldn't be shortening you couldn't be shortening he couldn't be shortening we couldn't be shortening you couldn't be shortening they couldn't be shortening
Perfect
I couldn't have shortened you couldn't have shortened he couldn't have shortened we couldn't have shortened you couldn't have shortened they couldn't have shortened
V-ing perfect
I couldn't have been shortening you couldn't have been shortening he couldn't have been shortening we couldn't have been shortening you couldn't have been shortening they couldn't have been shortening
Interrogative form
Simple form
could I shorten? could you shorten? could he shorten? could we shorten? could you shorten? could they shorten?
V-ing form
could I be shortening? could you be shortening? could he be shortening? could we be shortening? could you be shortening? could they be shortening?
Perfect
could I have shortened? could you have shortened? could he have shortened? could we have shortened? could you have shortened? could they have shortened?
V-ing perfect
could I have been shortening? could you have been shortening? could he have been shortening? could we have been shortening? could you have been shortening? could they have been shortening?
Interro-negative form
Simple form
couldn't I shorten? couldn't you shorten? couldn't he shorten? couldn't we shorten? couldn't you shorten? couldn't they shorten?
V-ing form
couldn't I be shortening? couldn't you be shortening? couldn't he be shortening? couldn't we be shortening? couldn't you be shortening? couldn't they be shortening?
Perfect
couldn't I have shortened? couldn't you have shortened? couldn't he have shortened? couldn't we have shortened? couldn't you have shortened? couldn't they have shortened?
V-ing perfect
couldn't I have been shortening? couldn't you have been shortening? couldn't he have been shortening? couldn't we have been shortening? couldn't you have been shortening? couldn't they have been shortening?