The vocative case is used to indicate direct address (i.e., to show when you are talking to somebody or something directly).
In English, nouns in the vocative case are offset using commas. For example:
I know, Stephen.
(Stephen is in the vocative case. As he is being addressed directly, his name is offset with a comma.)
Compare the sentence above with this one:
I know Stephen.
(There is no comma here. Stephen is now the direct object of the verb know.)
More Examples of the Vocative Case
Here are some more examples of nouns in the vocative case (shaded):
Where have you been, Charlie?
Jonathan, do not forget your shower gel.
And that, your Honor, concludes our case.
Do me a favour, Kev, and
ask Tim to stop bleating about the wind.
I've met your sister, Simon.
The vocative case applies to nouns and noun phrases. It is used most frequently with proper nouns (the specific names of things, e.g., Simon, Rover), but it is also used with common nouns (names for things, e.g., man, dog). For example:
Where have you been, you little adventurer?
(The word adventurer is a common noun. The term you little adventurer is a noun phrase, i.e., a group of words playing the role of a noun.)