Note: Most people would probably agree that commas are the most confusing punctuation marks because there are so many rules dictating when and how they should be used. I will not attempt to list all the comma rules on this page (there are some excellent sites listed on the Resources page that explain all the rules of comma usage); instead–in keeping with the theme of this site–I will highlight a few of the errors that I’ve observed to be the most common.
When combining two sentences into one compound sentence, a comma and a coordinating conjunction (and, but, or, nor, for, so, yet) are used.
Example 1: The festival was to be held today, but it was canceled due to the rainy weather. (comma and coordinating conjunction)