can stiffen
Regular verb: stiffen - stiffened- stiffened

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

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

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

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