Part of the problem is that each of these words may be used as a preposition. Only “besides” is also an adverb.
As a preposition, “beside” means “next to” as in “sitting next to.”
- Elizabeth was sitting beside her older sister until the whistle blew.
- The bulldogger’s horse ran beside the steer so his rider could drop off and wrestle the steer to the ground.
- If the cake had been beside the pie, the dog would have eaten both.
As a preposition, “besides” (with a final “s”) means “in addition to” or “except/other than.” Context explains the difference.
Examples as a preposition:
- Besides algebra, freshmen are required to take English composition and a science.
- Mathematics is a group of classes that, besides requiring a certain way of thinking, expects memorization of formulas and theorems.
- Everyone in the class besides Henrietta understood the Pythagorean Theorem.
- Nobody besides a special tutor could help her with her homework.
“Besides” may also be used as an adverb meaning “also” or “furthermore.”
Examples as an adverb:
- “Besides,” she complained, “I know I will never use this stuff.”
- The tutor offered her a pizza besides, as an incentive to come to his session.
- She shook her head. “I can’t learn this stuff, and besides, I don’t like pizza.”