stiffen vs modify

stiffen and modify both are verbs.

Word	Noun	Adjective	Verb	Adverb
stiffen	No	No	Yes	No
modify	No	No	Yes	No

As verbs, modify is a hypernym of stiffen; that is, modify is a word with a broader meaning than stiffen:

Other hypernyms of stiffen include alter, change.

stiffen (verb)	modify (verb)
become stiff or stiffer	make less severe or harsh or extreme
make stiff or stiffer	add a modifier to a constituent
severely restrict in scope or extent	cause to change; make different; cause a transformation