The word "jew" contains a number of elements that are offensive.
"Jew" can be used as an insult to connote being miserly (think, for example, of Shylock) or even to mean a cheat.
Thus to call someone "a jew" is to insult them. No other form of the work jew carries these additional meanings (jewish, jews) so it is appropriate and inoffensive to use them.
A Jew and Jews
The word "jew" contains a number of elements that are offensive.
"Jew" can be used as an insult to connote being miserly (think, for example, of Shylock) or even to mean a cheat.
Thus to call someone "a jew" is to insult them. No other form of the work jew carries these additional meanings (jewish, jews) so it is appropriate and inoffensive to use them.