Panis, at least up to this point, is a subject, in the nominative case. If you were to use an English sentence with it, it would be something like "The bread (panis) tastes good." Panem, on the other hand, is the accusative case, so it's the direct object. An English sentence demonstrating that is "I eat bread (panem)". Cases aren't really an easy thing to get used to for native speakers of languages that don't use them, but it'll come in time.