Learn Polish: Musimy szkolić odważnych żołnierzy.

You have a great eye! In Polish, for virile nouns (masculine nouns referring to people, like żołnierz), the accusative plural is identical to the genitive plural. Because they are the direct object of szkolić, they are technically in the accusative case here, but they borrow the genitive forms odważnych and żołnierzy.

How do we know to use the -y ending for the plural of żołnierz instead of -ów?

The singular dictionary form is żołnierz. Because it ends in -rz, which functions as a soft consonant in Polish grammar, the genitive and accusative plural takes the -y ending. Most hard-stem masculine nouns would take -ów instead.

What would odważnych look like if we were talking about animals instead of people?

If the noun isn't a masculine person, it belongs to the non-virile group. For non-virile nouns, the accusative plural looks exactly like the nominative plural. So, if we were training brave dogs, we would say szkolić odważne psy. We only use the -ych ending here because soldiers are masculine people.

Why use the imperfective verb szkolić rather than a perfective one?