when do we use which?

Hallo, danke für deine Frage. You use "der nächste" when referring to something or someone who is masculine and "die nächste" when referring to things or people that are feminine. For example: der nächste Tag - the next day (der Tag is masculine) die nächste Woche - the next week (die Woche is feminine) Does that make sense? All the best Nathalie

a further question: is it terribly wrong to mix up der and die with words, or is that more a grammatical thing, like the split infinitive in English. No one, except English teachers, would care about a split infinitive, so is getting gender things like der, die, das a hugh error?