Word order
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.