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