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 couldn't stiffen you couldn't stiffen he couldn't stiffen we couldn't stiffen you couldn't stiffen they couldn't stiffen	I couldn't be stiffening you couldn't be stiffening he couldn't be stiffening we couldn't be stiffening you couldn't be stiffening they couldn't be stiffening
Perfect	V-ing perfect
I couldn't have stiffened you couldn't have stiffened he couldn't have stiffened we couldn't have stiffened you couldn't have stiffened they couldn't have stiffened	I couldn't have been stiffening you couldn't have been stiffening he couldn't have been stiffening we couldn't have been stiffening you couldn't have been stiffening they couldn't 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
couldn't I stiffen? couldn't you stiffen? couldn't he stiffen? couldn't we stiffen? couldn't you stiffen? couldn't they stiffen?	couldn't I be stiffening? couldn't you be stiffening? couldn't he be stiffening? couldn't we be stiffening? couldn't you be stiffening? couldn't they be stiffening?
Perfect	V-ing perfect
couldn't I have stiffened? couldn't you have stiffened? couldn't he have stiffened? couldn't we have stiffened? couldn't you have stiffened? couldn't they have stiffened?	couldn't I have been stiffening? couldn't you have been stiffening? couldn't he have been stiffening? couldn't we have been stiffening? couldn't you have been stiffening? couldn't they have been stiffening?