straighten vs unbend

straighten and unbend both are verbs.

Word	Noun	Adjective	Verb	Adverb
straighten	No	No	Yes	No
unbend	No	No	Yes	No

As verbs, unbend is a hyponym of straighten; that is, unbend is a word with a more specific, narrower meaning than straighten:

Other hyponyms of straighten include untwist, extend, channelise, channelize, untwine, unweave, comb.

As verbs, straighten and unbend are synonyms defined as:

straighten (verb)	unbend (verb)
put (things or places) in order	release from mental strain, tension, or formality
make straight or straighter	become less tense, rest, or take one's ease
make straight	make less taut
straighten by unrolling	free from flexure
straighten up or out; make straight	unfasten, as a sail, from a spar or a stay
get up from a sitting or slouching position	straighten up or out; make straight