"Je mange la pomme ronde."

Translation: I am eating the round apple.

[Duomail]

should the "e" of "ronde" sound?

[MimicMethod.com]

yes

[JanetHudgins]

when does the adjective come before the noun, and when after

[Proprietous]

In general, adjectives always go after the noun, except for when you're talking about:
Beauty,
Age,
Goodness, or
Size.
"BAGS" is a helpful mnemonic device to remember this. After you practice for a while, it'll start to "feel" right.

[cuddlekits]

It's not very clear cut.
Usually they'll go after, I'd say around 80% of the time. Sometimes basic adjectives can go before the noun, such as "petit" (small) and "grand" (big). And adjectives with function rather than a descriptive purpose would go before, such as "plusieurs" (several). And they can go before the noun for emphasis, such as the word "sale" meaning unpleasant or dirty.
Colour adjectives and adjectives denoting shape however, will always come afterwards.

[n6zs]

https://www.thoughtco.com/introduction-to-french-adjectives-1368789

[Idraote]

assis à la table ronde près du roi Arthur

[Gilgoose]

I feel like I'm too far into French to be asking this, but how would you say something like the red, round apple? Would it be "la pomme rouge et ronde," "la pomme rouge ronde," or something like "la pomme est rouge et ronde"?

[n6zs]

If the two adjectives are ones which follow the noun, separate them with "et", i.e., la pomme ronde et rouge. Sometimes one may be a BANGS adjective and the other not, in which case: le grosse pomme rouge. https://www.thoughtco.com/introduction-to-french-adjectives-1368789

[nectarivorous]

Not actually that weird a statement - most apples are not actually round, they are apple-shaped. It is more than likely a Nashi...