What you’ll learn to do: critique the use of commas
Perhaps the best and most instructive way to approach the comma is to remember its chief function: it is a separator. Once you know this, the next step is to determine what sorts of things generally require separation. This includes most transition words, descriptive words or phrases, adjacent items, and complete ideas (complete ideas contain both a subject and a verb). Commas are also used to separate similar items in lists.