should stiffen
Regular verb: stiffen - stiffened- stiffened

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I'd stiffen
you'd stiffen
he'd stiffen
we'd stiffen
you'd stiffen
they'd stiffen 	I'd be stiffening
you'd be stiffening
he'd be stiffening
we'd be stiffening
you'd be stiffening
they'd be stiffening	I should've stiffened
you should've stiffened
he should've stiffened
we should've stiffened
you should've stiffened
they should've stiffened	I should've been stiffening
you should've been stiffening
he should've been stiffening
we should've been stiffening
you should've been stiffening
they should've been stiffening	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I shouldn't stiffen
you shouldn't stiffen
he shouldn't stiffen
we shouldn't stiffen
you shouldn't stiffen
they shouldn't stiffen 	I shouldn't be stiffening
you shouldn't be stiffening
he shouldn't be stiffening
we shouldn't be stiffening
you shouldn't be stiffening
they shouldn't be stiffening	I shouldn't have stiffened
you shouldn't have stiffened
he shouldn't have stiffened
we shouldn't have stiffened
you shouldn't have stiffened
they shouldn't have stiffened	I shouldn't have been stiffening
you shouldn't have been stiffening
he shouldn't have been stiffening
we shouldn't have been stiffening
you shouldn't have been stiffening
they shouldn't have been stiffening	

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

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
shouldn't I stiffen?
shouldn't you stiffen?
shouldn't he stiffen?
shouldn't we stiffen?
shouldn't you stiffen?
shouldn't they stiffen? 	shouldn't I be stiffening?
shouldn't you be stiffening?
shouldn't he be stiffening?
shouldn't we be stiffening?
shouldn't you be stiffening?
shouldn't they be stiffening? 	shouldn't I have stiffened?
shouldn't you have stiffened?
shouldn't he have stiffened?
shouldn't we have stiffened?
shouldn't you have stiffened?
shouldn't they have stiffened? 	shouldn't I have been stiffening?
shouldn't you have been stiffening?
shouldn't he have been stiffening?
shouldn't we have been stiffening?
shouldn't you have been stiffening?
shouldn't they have been stiffening?