Watch this youtube video. It is clear and easy to understand. It generally covers periods, commas, quotation marks, and exclamation points. In what country do you think this video was made?
Comma Rules and an Exercise: Review the comma rules, then go to this link and do the exercise at the bottom of the page. Your work can be automatically scored. The written answers follow the sentences. http://www.myenglishteacher.net/usingcommas.html
Comma Review on the Goodwill site: http://www.gcflearnfree.org/grammar/commas/full