"Apples are good."
Translation:Pomoj estas bonaj.
You use it in the accusative case, that is, the object of the sentence. That's more-or-less the noun phrase that comes after the verb in a normal sentence. If I said "I eat apples", then you'd use "pomojn". But if you say "Apples are red", you use "pomoj." One exception: in "estas" you don't use the "-n" for either noun because it equates the subject and the object.