English verb conjugation
COULD SIGN

Regular verb: sign - signed - signed

Affirmation

Simple form	V-ing form
I could sign you could sign he could sign we could sign you could sign they could sign	I could be signing you could be signing he could be signing we could be signing you could be signing they could be signing
Perfect	V-ing perfect
I could have signed you could have signed he could have signed we could have signed you could have signed they could have signed	I could have been signing you could have been signing he could have been signing we could have been signing you could have been signing they could have been signing

Negation

Simple form	V-ing form
I couldn't sign you couldn't sign he couldn't sign we couldn't sign you couldn't sign they couldn't sign	I couldn't be signing you couldn't be signing he couldn't be signing we couldn't be signing you couldn't be signing they couldn't be signing
Perfect	V-ing perfect
I couldn't have signed you couldn't have signed he couldn't have signed we couldn't have signed you couldn't have signed they couldn't have signed	I couldn't have been signing you couldn't have been signing he couldn't have been signing we couldn't have been signing you couldn't have been signing they couldn't have been signing

Interrogative form

Simple form	V-ing form
could I sign? could you sign? could he sign? could we sign? could you sign? could they sign?	could I be signing? could you be signing? could he be signing? could we be signing? could you be signing? could they be signing?
Perfect	V-ing perfect
could I have signed? could you have signed? could he have signed? could we have signed? could you have signed? could they have signed?	could I have been signing? could you have been signing? could he have been signing? could we have been signing? could you have been signing? could they have been signing?

Interro-negative form

Simple form	V-ing form
couldn't I sign? couldn't you sign? couldn't he sign? couldn't we sign? couldn't you sign? couldn't they sign?	couldn't I be signing? couldn't you be signing? couldn't he be signing? couldn't we be signing? couldn't you be signing? couldn't they be signing?
Perfect	V-ing perfect
couldn't I have signed? couldn't you have signed? couldn't he have signed? couldn't we have signed? couldn't you have signed? couldn't they have signed?	couldn't I have been signing? couldn't you have been signing? couldn't he have been signing? couldn't we have been signing? couldn't you have been signing? couldn't they have been signing?