So this can be generalized as:
-Do not use apostrophes when pluralizing.
-Do use apostrophes when indicating any singular possessive. (My parent's car, if I had only one parent)
-Do use an apostrophe when indicating a plural possessive, but do not add a trailing s if the word already has an s for pluralization. (My parents' house, if I have more than one parent; The children's room.)


I was also taught to not use apostrophes on possessive pronouns.