slow vs retard

slow is not a noun while retard is a noun.

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

As verbs, slow and retard are synonyms defined as:

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

As verbs, retard is a hypernym of slow; that is, retard is a word with a broader meaning than slow:

Other hypernyms of slow include decelerate, slow, slow down, slow up.

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

slow (adjective)	retard (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

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

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