- An " 's" never makes a word plural. So please don't write stuff like "Case's of cold Red Bull for sale."
- An apostrophe serves a few purposes: signaling contractions (don't, can't, etc.), signaling possession (Mike's book, my parents' names), and, rarely, to indicate the shortening up of a word, usually to represent in writing an accent or unique use of word ('twas, buggin', " 'allo there, 'ow are you?" some Irish guy asked me).
- Though not officially an apostrophe rule, its misuse in "its" and "it's" deserves special mention. "It's" always means "it is" or "it has." "Its" always means "belonging to it." It's time for its uses to be clear.
There may be more uses as well as better explanations for the three above (those were off the top of my head). For now, though, class is dismissed.