Warning, this verb may not exist. Conjugation is given as example.
Affirmation
Simple form
I must stiffer you must stiffer he must stiffer we must stiffer you must stiffer they must stiffer
V-ing form
I must be stiffering you must be stiffering he must be stiffering we must be stiffering you must be stiffering they must be stiffering
Perfect
I must have stiffered you must have stiffered he must have stiffered we must have stiffered you must have stiffered they must have stiffered
V-ing perfect
I must have been stiffering you must have been stiffering he must have been stiffering we must have been stiffering you must have been stiffering they must have been stiffering
Negation
Simple form
I must not stiffer you must not stiffer he must not stiffer we must not stiffer you must not stiffer they must not stiffer
V-ing form
I must not be stiffering you must not be stiffering he must not be stiffering we must not be stiffering you must not be stiffering they must not be stiffering
Perfect
I must not have stiffered you must not have stiffered he must not have stiffered we must not have stiffered you must not have stiffered they must not have stiffered
V-ing perfect
I must not have been stiffering you must not have been stiffering he must not have been stiffering we must not have been stiffering you must not have been stiffering they must not have been stiffering
Interrogative form
Simple form
must I stiffer? must you stiffer? must he stiffer? must we stiffer? must you stiffer? must they stiffer?
V-ing form
must I be stiffering? must you be stiffering? must he be stiffering? must we be stiffering? must you be stiffering? must they be stiffering?
Perfect
must I have stiffered? must you have stiffered? must he have stiffered? must we have stiffered? must you have stiffered? must they have stiffered?
V-ing perfect
must I have been stiffering? must you have been stiffering? must he have been stiffering? must we have been stiffering? must you have been stiffering? must they have been stiffering?
Interro-negative form
Simple form
must I not stiffer? must you not stiffer? must he not stiffer? must we not stiffer? must you not stiffer? must they not stiffer?
V-ing form
must I not be stiffering? must you not be stiffering? must he not be stiffering? must we not be stiffering? must you not be stiffering? must they not be stiffering?
Perfect
must I not have stiffered? must you not have stiffered? must he not have stiffered? must we not have stiffered? must you not have stiffered? must they not have stiffered?
V-ing perfect
must I not have been stiffering? must you not have been stiffering? must he not have been stiffering? must we not have been stiffering? must you not have been stiffering? must they not have been stiffering?