Error # 11: Incorrect Comma with Essential & Non-essential Element

Mini-lesson
http://owl.english.purdue.edu/handouts/grammar/g_commaess.html

Practice Quizzes:
http://owl.english.purdue.edu/handouts/interact/g_commaessEX1.html
http://owl.english.purdue.edu/handouts/interact/g_commaessEX2.html
http://owl.english.purdue.edu/handouts/interact/g_commaessEX3.html