The Object Case in English is used for syntactic relationships that require either the Accusative or the Dative Case in Latin. This changes the way transitive and intransitive verbs may be used in the passive voice.
In Latin, the passive voice is defined as the verb form in which the accusative object of the active form is made the subject. Thus, the content of the active expression "vides me" may be expressed passively as "ego videor (a te)" or the content of "video Caesarem" as "Caesar videtur (a me)". The same can be said of the English object case (in the accusative function): "You see me" = "I am seen (by you)" and "I see Caesar" = "Caesar is seen (by me)". So far, the languages are parallel.
For Latin, the rule means that a dative indirect object cannot become the subject of a passive verb: librum tibi do ("I give to you a book") has only one passive equivalent: liber tibi datur ("A book is given to you"). English, however, does not formally distinguish the Dative from the Accusative. As a result, the rule that allows the English object to become the subject of a passive verb includes the object case in the dative function: "you are given a book" is a possible passive in English, but would be impossible in Latin. Similarly, "You are sung a song."
The extension of the rule for passive verbs to the dative function in English shows the extent to which the dative is no longer a distinct case in English. However, the conflation of the accusative and the dative is not complete. This is indicated by the fact that, while you can make a complete sentence out of the passive transitive verb + accusative object ("A book is given."), you cannot make a complete sentence out of the passive transitive verb + dative object ("You are given.") -- or, rather, when you do, the subject is interpreted as a direct object of a transitive verb.