slow vs slow down

slow is an adjective but slow down is not an adjective.

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

As verbs, slow and slow down are synonyms defined as:

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

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

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

As verbs, slow and slow down are synonyms defined as:

Other synonyms of slow include slack, slacken, slow up.

As verbs, slow and slow down are synonyms defined as:

Other synonyms of slow include slow up.

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

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

slow (adjective)	slow down (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)	slow down (verb)
lose velocity; move more slowly	become less tense, rest, or take one's ease
become slow or slower	lose velocity; move more slowly
cause to proceed more slowly	become slow or slower
	reduce the speed of
	cause to proceed more slowly

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