Historically, the vocative case was an element of the Indoeuropean system of cases, and existed in Latin, Sanskrit, and Classical Greek. Although it has been lost by many modern Indoeuropean languages, some languages have retained the vocative case to this day. Examples are Modern Greek and Slavic languages such as Polish, Czech, Serbo-Croatic, Ukrainian, and - to a lesser extent - Russian.
The vocative case in various languages
In Latin the vocative case of a noun is the same as the nominative, except for masculine singular second declension nouns that have the ending -us in the nominative case. An example would be the famous line from Shakespeare, "Et tu, Brute?" ("And you, Brutus?", commonly translated as "You too, Brutus?"), where Brute is the vocative case, whilst Brutus would be the nominative case. When Latin names in the vocative case are translated into English, the nominative case is usually used, as English simply uses the nominative case for vocative expressions but sets them off from the rest of the sentences with pauses as interjections (rendered in writing as commas) (see below).
Four historical Indoeuropean languages
on the example of the word for "wolf".
Notes on notation: The elements separated with hyphens denote the stem, the so-called theme vowel of the case and the actual suffix. The symbol "Ø" means that there is no suffix in a place where other cases may have one. In Latin, e.g., the nominative case is lupus and the vocative case is lupe!, whereas the accusative case is lupum. The asterisk in front of the Indoeuropean words means that they are merely hypothetical reconstructions, not based on any written sources.
In Polish, unlike in Latin, the vocative is almost always different from the nominative case and is being formed according to a complex grammatical pattern. Here are some examples.
There are very few exceptions where the vocative case can be replaced with the vocative (e.g. Ewa!), but normally it is used even in informal speech.
Vocative-like expressions in English
In English the vocative case is not marked, but English syntax performs a similar function; witness: "John, could you come here?" or "I don't think so, John", where "John" is neither subject nor object of the verb, but rather indicates the person to whom the statement is being addressed. Other examples for vocative markers are O Death, where is thy victory, or Hey, you!. These vocative expressions are are usually classified as interjections and can occur in any clause, irrespective of mood. Some examples: