"The girl is eating with the boy."
Translation:Das Mädchen isst mit dem Jungen.
Dem, because the mit preposition is dative case, and JungeN, because it's a weak noun. See http://www.vistawide.com/german/grammar/german_nouns03.htm
Good question, it is slightly better grammer to use jungen but junge would also be understood here in germany.
As if having four declinations and three genders was not difficult enough, I now come across weak nouns!
Junge is a weak masculine noun and when it's in the accusative or dative case you add an 'n' to the end of the noun. So mit dem Jungen is just with the boy. If you wanted "with the boys" you would say mit den Jungen, "den" being the dative plural.
Hmmm can someone explain why mit dem jungen isst das madchen is wrong