If you want to write well in English it is essential to learn how to use commas correctly. This will make a big difference in how your ideas flow and will certainly impress an examiner, who is, after all, only human, and must get heartily sick of compositions with terrible punctuation. So here’s a fine explanation of how and when to use them complete with a quiz at the bottom:

http://grammar.ccc.commnet.edu/grammar/commas.htm