I could stiffen you could stiffen he could stiffen we could stiffen you could stiffen they could stiffen
V-ing form
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
I could have stiffened you could have stiffened he could have stiffened we could have stiffened you could have stiffened they could have stiffened
V-ing perfect
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
I couldn't stiffen you couldn't stiffen he couldn't stiffen we couldn't stiffen you couldn't stiffen they couldn't stiffen
V-ing form
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
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
V-ing perfect
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
could I stiffen? could you stiffen? could he stiffen? could we stiffen? could you stiffen? could they stiffen?
V-ing form
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
could I have stiffened? could you have stiffened? could he have stiffened? could we have stiffened? could you have stiffened? could they have stiffened?
V-ing perfect
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
couldn't I stiffen? couldn't you stiffen? couldn't he stiffen? couldn't we stiffen? couldn't you stiffen? couldn't they stiffen?
V-ing form
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
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?
V-ing perfect
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?