sugar vs sweeten

sugar is a noun but sweeten is not a noun.

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

As verbs, sweeten is a hypernym of sugar; that is, sweeten is a word with a broader meaning than sugar:

Other hypernyms of sugar include dulcify, dulcorate, edulcorate.

sugar (noun)	sweeten (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

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