sing vs hymn

sing is not a noun while hymn is a noun.

Word	Noun	Adjective	Verb	Adverb
sing	No	No	Yes	No
hymn	Yes	No	Yes	No

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

Other hyponyms of sing include solmizate, troll, carol, madrigal.

sing (noun)	hymn (noun)
	a song of praise (to God or to a saint or to a nation)

sing (verb)	hymn (verb)
make a whining, ringing, or whistling sound	praise by singing a hymn
divulge confidential information or secrets	sing a hymn
to make melodious sounds
produce tones with the voice
deliver by singing