"I like English more than math."
Translation:Ich mag Englisch lieber als Mathematik.
Lieber is no verb, you can't say 'Ich lieber/e'. Lieb is an adjective. But if you use lieber in this sentence, its the comparative of gern. So in this sentence, lieber is an adverb, because ich MAG ... lieber als.... Its an Adjektiv, whenever sein, bin, bist etc. appears. And that is impossible with lieber. You can't say 'Ich bin lieber/du bist lieber'