Questions & Answers about Hún syngur betur en við.
Betur is the comparative form of the adverb vel (“well”). Adverbs modify verbs, so to say “sings better” you need betur.
Betri is the comparative form of the adjective góður (“good”), used to modify nouns (e.g. betri söngvari “a better singer”). You wouldn’t use betri to modify syngur (“sings”).
In comparative constructions en means “than.” The structure is:
Subject + verb + comparative adverb + en + second element.
So Hún syngur betur en við literally is “She sings better than we [do].” Outside comparisons, en can indeed mean “and” or “but,” but here its role is strictly comparative.