Learn Portuguese (Portugal): O cão velho já não rói ossos.

In Portuguese, já não (meaning "no longer" or "not anymore") almost always goes immediately before the verb. While in English you might put "anymore" at the end of the sentence, Portuguese structures it as "The dog already not gnaws bones" (O cão velho já não rói ossos).

What verb is rói from, and why does it have an accent mark?

It comes from the verb roer (to gnaw or chew). Rói is the third-person singular present tense form (he/she/it gnaws). The acute accent is required to tell you to stress the "o" and pronounce it with an open sound. Without the accent, the normal pronunciation rules would make it sound completely different.

Could I say O velho cão instead of O cão velho?

You could, but it changes the meaning slightly. Placing the adjective after the noun (cão velho) is the standard way to describe a physical trait, meaning the dog is actually advanced in years. Placing it before the noun (velho cão) makes it more subjective or emotional, translating closer to "long-time dog" or "old companion."

Is there a pronunciation change when going from singular osso to plural ossos?

Yes! The singular word osso (bone) is pronounced with a closed "o" sound (similar to "boat"). However, in the plural ossos, that first "o" opens up to sound more like the "o" in the English word "boss". This vowel shift in the plural is called metafonia and happens with several Portuguese words.