retard vs slow

retard is a noun but slow is not a noun.

Word	Noun	Adjective	Verb	Adverb
retard	Yes	No	Yes	No
slow	No	Yes	Yes	Yes

As verbs, retard and slow are synonyms defined as:

Other synonyms of retard include decelerate, slow down, slow up.

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

Other hyponyms of retard include slow down, slow up, delay, detain, hold up.

retard (noun)	slow (noun)
a person of subnormal intelligence

retard (adjective)	slow (adjective)
	not moving quickly; taking a comparatively long time
	at a slow tempo
	(used of timepieces) indicating a time earlier than the correct time
	(of business) not active or brisk
	slow to learn or understand; lacking intellectual acuity
	so lacking in interest as to cause mental weariness

retard (verb)	slow (verb)
lose velocity; move more slowly	lose velocity; move more slowly
cause to move more slowly or operate at a slower rate	become slow or slower
slow the growth or development of	cause to proceed more slowly
be delayed

retard (adverb)	slow (adverb)
	without speed (`slow' is sometimes used informally for `slowly')
	showing a time that is earlier than the actual time