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