English verb conjugation
MUST STIFFEN

Regular verb: stiffen - stiffened - stiffened

Affirmation

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