English verb conjugation
COULD STIFFEN

Regular verb: stiffen - stiffened - stiffened

Affirmation

Simple form	V-ing form
I could stiffen you could stiffen he could stiffen we could stiffen you could stiffen they could stiffen	I could be stiffening you could be stiffening he could be stiffening we could be stiffening you could be stiffening they could be stiffening
Perfect	V-ing perfect
I could have stiffened you could have stiffened he could have stiffened we could have stiffened you could have stiffened they could have stiffened	I could have been stiffening you could have been stiffening he could have been stiffening we could have been stiffening you could have been stiffening they could have been stiffening

Negation

Simple form	V-ing form
I could not stiffen you could not stiffen he could not stiffen we could not stiffen you could not stiffen they could not stiffen	I could not be stiffening you could not be stiffening he could not be stiffening we could not be stiffening you could not be stiffening they could not be stiffening
Perfect	V-ing perfect
I could not have stiffened you could not have stiffened he could not have stiffened we could not have stiffened you could not have stiffened they could not have stiffened	I could not have been stiffening you could not have been stiffening he could not have been stiffening we could not have been stiffening you could not have been stiffening they could not have been stiffening

Interrogative form

Simple form	V-ing form
could I stiffen? could you stiffen? could he stiffen? could we stiffen? could you stiffen? could they stiffen?	could I be stiffening? could you be stiffening? could he be stiffening? could we be stiffening? could you be stiffening? could they be stiffening?
Perfect	V-ing perfect
could I have stiffened? could you have stiffened? could he have stiffened? could we have stiffened? could you have stiffened? could they have stiffened?	could I have been stiffening? could you have been stiffening? could he have been stiffening? could we have been stiffening? could you have been stiffening? could they have been stiffening?

Interro-negative form

Simple form	V-ing form
could I not stiffen? could you not stiffen? could he not stiffen? could we not stiffen? could you not stiffen? could they not stiffen?	could I not be stiffening? could you not be stiffening? could he not be stiffening? could we not be stiffening? could you not be stiffening? could they not be stiffening?
Perfect	V-ing perfect
could I not have stiffened? could you not have stiffened? could he not have stiffened? could we not have stiffened? could you not have stiffened? could they not have stiffened?	could I not have been stiffening? could you not have been stiffening? could he not have been stiffening? could we not have been stiffening? could you not have been stiffening? could they not have been stiffening?