Learn Polish: Dziecko lubi młodego nauczyciela. - The child likes the young teacher.

Nauczyciel (teacher) is a masculine animate noun, meaning it's a masculine word that refers to a living person or animal. Because the teacher is the direct object of the sentence (the person being liked), it must take the accusative case. For masculine animate nouns, we add -a to the end, making it nauczyciela.

Why does the adjective młodego end in -ego?

Adjectives always have to match the grammatical case and gender of the noun they describe. Because nauczyciela is in the accusative case for a masculine animate noun, the base adjective młody (young) must also change to its matching accusative form. For masculine animate adjectives, the ending changes to -ego, giving us młodego.

If we flipped the sentence so the teacher likes the child, would dziecko change its ending?

No, it would stay exactly the same! Dziecko is a neuter noun. A handy rule in Polish is that for neuter nouns, the accusative form (when it's the object) is identical to the nominative form (when it's the subject). So, "The young teacher likes the child" would be Młody nauczyciel lubi dziecko.

Why do we use lubi instead of lubię or lubisz?