sustain vs back

sustain is not a noun while back is a noun.

Word	Noun	Adjective	Verb	Adverb
sustain	No	No	Yes	No
back	Yes	Yes	Yes	Yes

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

Other hyponyms of sustain include back up, vouch, verify, demonstrate, establish, prove, shew, show, document, validate.

sustain (noun)	back (noun)
	(American football) the position of a player on a football team who is stationed behind the line of scrimmage
	the side that goes last or is not normally seen
	a support that you can lean against while sitting
	the part of a garment that covers the back of your body
	the protective covering on the front, back, and spine of a book
	the posterior part of a human (or animal) body from the neck to the end of the spine
	the series of vertebrae forming the axis of the skeleton and protecting the spinal cord
	the part of something that is furthest from the normal viewer
	(football) a person who plays in the backfield

sustain (adjective)	back (adjective)
	related to or located at the back
	of an earlier date
	located at or near the back of an animal

sustain (verb)	back (verb)
undergo (as of injuries and illnesses)	establish as valid or genuine
establish or strengthen as with new evidence or facts	strengthen by providing with a back or backing
admit as valid	shift to a counterclockwise direction
supply with necessities and support	place a bet on
provide with nourishment	travel backward
be the physical support of; carry the weight of	cause to travel backward
lengthen or extend in duration or space	support financial backing for
	be behind; approve of
	give support or one's approval to
	be in back of

sustain (adverb)	back (adverb)
	at or to or toward the back or rear
	in or to or toward a past time
	in repayment or retaliation
	in or to or toward a former location
	in or to or toward an original condition
	in reply