"lieber" indicates preference where there is a choice -> I'd rather; to prefer something to sth else "Ich trinke lieber Tee als Kaffee" (I prefer tea to coffee; I'd rather have/drink tea than coffee) "Ich mag Katzen lieber als Hunde" (I prefer cats to dogs) "mag" should only be used with nouns: "I like chocolate/cats/you" -> Ich mag Schokolade/Katzen/Dich (gern) "gern" CAN be added to "mag" to lend a little emphasis. Otherwise, "gern" is used with verbs -> I like -ing Ich spiele gern Schach. -> I like playing Schach. Ich lerne gern Sprachen. -> I like learning languages. NOT: "Ich mag Schach spielen" Hope that helps?

lieber is a comparative form of gern. (gern-lieber-am liebsten = gladly-more gladly-the most gladly) mag comes from "mögen" which means "to like" so it's a verb. For example: Ich mag dieses Auto. （我喜欢这辆车。）