answer:Use them to separate the parts of sentences, which are referring to different or tangential things. For example, when clarifying a point or mentioning an extra piece of information, such as a condition or an example like this one. Whenever you would pause in natural flow of speech. That should get you most of the non-mandatory-but-extremely-beneficial-for-the-reader commas, and the other instances have rules, such as “always before ‘which’, never before ‘that’”, etc.