They're different cases ("du" nominative, "dir" dative), so it depends on the grammatical context of the pronoun. If the previous sentence used a nominative pronoun, you have to use "du", if there was a dative one, it has to be "dir" (and there's also "dich" for accusative). – Ich (←nominative) habe Hunger? Und du? – Mir (←dative) geht es gut. Und dir?

Hey, I'm not the best with german grammar,.. but! You are using du / dir to talk to friends, family, etc. Sie / Ihnen is the polite form of it. Did you have a look at nominative, genetive, accusative and dative yet? dir: dative -> Ich gebe dir ein Geschenk. You can ask for a dative case with the German word "Wem?" example: Wem gibst du das Geschenk? du: nominative -> Du kommst aus Korea? As for nominative you can use "Wer?" example: Wer kommt aus Korea? I hope this is correct. German is my native language, but I never really have to think about all the cases. They just come to me naturally :D

Danke :)