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