There is no short answer to this question, Luis because there are many examples. Usually we use the comma to separate different parts of the sentence, or to show a place where we make a pause, so as to make the meaning clearer and avoid confusion in meaning.
Start with this link
https://owl.english.purdue.edu/owl/owlprint/607/
and then see if this helps you to understand.
May 9, 2016
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