These two do almost opposite work. One takes something in, the other leaves it out. Which makes the mistake unusually jarring when it lands. "Everyone accept Maria" and "everyone except Maria" describe entirely different situations, and a reader has to stop and work out which one you meant.
A verb. To receive something offered, or to agree to something.
Usually a preposition meaning "not including." Occasionally a verb meaning to exclude.
Quick way to remember
Try replacing the word with "take." If "take" fits — you take an offer, you accept an offer — use "accept." If the sentence means "but not," use "except," which shares its start with "exclude."