Cambridge Dictionary gave the following rules that will help us learn how to use “beside” or “besides”

A. Beside is a preposition. It means ‘at the side of’ or ‘next to’. It is rather formal:
Example: He would like to take a photograph of us. Would you come and sit beside me?
B. Besides is a preposition or a linking adverb. It means ‘in addition to’ or ‘also’:
Example: What other types of music do you like besides classical?
As a linking adverb, we usually put a comma before and after besides in writing:
Example: I don’t think going for a walk is a good idea. It’s quite cold, and, besides, it’s getting late and we don’t want to be out in the dark.
To read more about this post, please click here.
To take the quiz, please click here.