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