Learn French: Les jeunes joueurs jouent activement. - The young players play actively.

To make an adverb out of a French adjective, you generally start with its feminine form. The masculine adjective for active is actif, but its feminine form is active. Add -ment to that feminine form, and you get activement (actively).

Why is the adjective jeunes placed before joueurs?

Most French adjectives go after the noun, but jeune (young) is an exception. It belongs to a small group of common adjectives describing Beauty, Age, Goodness, and Size (sometimes remembered as BAGS) that always come before the noun they describe.

Do I pronounce the -ent at the end of jouent?

No, the -ent ending on third-person plural verbs is completely silent in French. You pronounce jouent exactly the same way you pronounce the singular form joue.

How do I know if joueurs is masculine or feminine here?

The noun joueur (player) is masculine. If the players were all female, the sentence would use the feminine plural noun joueuses, and the sentence would be Les jeunes joueuses jouent activement.