in other languages, sentences can use other word orders. consider "robert opens the door". in english, changing the word order to "the door opens robert" would change the meaning of the sentence. in latin, however, "robertus ianuam aperit" and "ianuam robertus aperit" mean the same. "ianuam" is in the accusative case and so it is the direct object and "robertus" the subject. changing the cases of the words, however, to "robertem ianua aperit" would change the meaning of the sentence: "ianua" is now in the nominative case and so it is the subject and "robertum" the object.