retarded vs backward

retarded is a noun but backward is not a noun.

Word	Noun	Adjective	Verb	Adverb
retarded	Yes	Yes	No	No
backward	No	Yes	No	Yes

retarded (noun)	backward (noun)
people collectively who are mentally retarded

retarded (adjective)	backward (adjective)
relatively slow in mental or emotional or physical development	directed or facing toward the back or rear
	(used of temperament or behavior) marked by a retiring nature
	retarded in intellectual development
	having made less than normal progress

retarded (adverb)	backward (adverb)
	in a manner or order or direction the reverse of normal
	in or to or toward a past time
	at or to or toward the back or rear