Commas tell readers where to take a breath or change the tone of their voice when reading a text.
They are used to separate distinct parts of a sentence, add parts to a sentence, or to make one complex sentence out of two or more simple sentences. Commas indicate that the sentence is changing direction, but that it is not yet complete. Information can be added at the beginning of a sentence, in the middle of a sentence, or at the end of a sentence. Think of these three forms as introductory phrase markers. Having slept all day, Susan was awake all night.