must stiffen
Regular verb: stiffen - stiffened- stiffened

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

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

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

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