BESIDE and BESIDES often get confused. Both are prepositions, but they are used differently. Besides can also be used as an adverb, but beside cannot.
BESIDE: a preposition used to determine the spatial relationship between two objects. It means “at the side of” or “next to.” For instance, “He sat beside the bed while his sister was sleeping.”
BESIDES: as a preposition, it means “in addition to” or “except.” For example, “She wants to play other sports besides tennis.”
BESIDES: as an adverb, it means “also” or “in addition to.”
The movie looks good, and besides, it’s my birthday so I want to go.
TRICK: BESIDE can only be a preposition and usually relates to space: at the side of/next to. BESIDES has an additional letter S, and it can mean “in addition to.”