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 could not sign you could not sign he could not sign we could not sign you could not sign they could not sign	I could not be signing you could not be signing he could not be signing we could not be signing you could not be signing they could not be signing
Perfect	V-ing perfect
I could not have signed you could not have signed he could not have signed we could not have signed you could not have signed they could not have signed	I could not have been signing you could not have been signing he could not have been signing we could not have been signing you could not have been signing they could not 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
could I not sign? could you not sign? could he not sign? could we not sign? could you not sign? could they not sign?	could I not be signing? could you not be signing? could he not be signing? could we not be signing? could you not be signing? could they not be signing?
Perfect	V-ing perfect
could I not have signed? could you not have signed? could he not have signed? could we not have signed? could you not have signed? could they not have signed?	could I not have been signing? could you not have been signing? could he not have been signing? could we not have been signing? could you not have been signing? could they not have been signing?