I was reminded today of a droll example of the difference between a restrictive and a nonrestrictive clause:
My boyfriend who plays the piano is taking me to dinner.
My boyfriend, who plays the piano, is taking me to dinner.
How many boyfriends does each speaker have?
Source:
a Fulbright scholar (and native speaker of Arabic) assisting Fran Biscoglia in Applied English Grammar (English 402) Fall semester 2011