nail down vs nail

nail down is not a noun while nail is a noun.

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

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

Other synonyms of nail down include peg.

nail down (noun)	nail (noun)
	a thin pointed piece of metal that is hammered into materials as a fastener
	horny plate covering and protecting part of the dorsal surface of the digits
	a former unit of length for cloth equal to 1/16 of a yard

nail down (verb)	nail (verb)
make final; put the last touches on; put into final form	complete a pass
define clearly	attach something somewhere by means of nails
succeed in obtaining a position	take into custody
	hit with great force
	locate exactly
	succeed at easily
	succeed in obtaining a position