Learn Italian: Le ossa del braccio sono rotte.

L'osso (the bone) is one of a special group of Italian nouns that are masculine in the singular but become feminine in the plural. Because it becomes feminine plural, its definite article is le, and its ending changes to -a: le ossa. Many body parts do this!

Does rotte end in -e because it's agreeing with le ossa?

Exactly! Even though the singular word l'osso is masculine, the plural le ossa acts as a feminine plural noun. Therefore, the adjective rotto (broken) must change to its feminine plural form, rotte, to match it.

Why does the Italian sentence say del braccio instead of just combining the words like "ossa braccio"?

Italian doesn't stack nouns next to each other like English does (as in "arm bones"). Instead, you have to say "the bones of the arm". To do this, you combine di (of) with the article il (the) to make del, giving you le ossa del braccio.

Since we are learning about irregular plurals, why isn't it delle braccia here?