sweeten vs sugar

sweeten is not a noun while sugar is a noun.

Word	Noun	Adjective	Verb	Adverb
sweeten	No	No	Yes	No
sugar	Yes	No	Yes	No

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

Other hyponyms of sweeten include honey, saccharify, candy, glaze, sugarcoat, mull.

sweeten (noun)	sugar (noun)
	a white crystalline carbohydrate used as a sweetener and preservative
	informal terms for money
	an essential structural component of living cells and source of energy for animals; includes simple sugars with small molecules as well as macromolecular substances; are classified according to the number of monosaccharide groups they contain

sweeten (verb)	sugar (verb)
make sweeter, more pleasant, or more agreeable	sweeten with sugar
make sweeter in taste