Who owns what? An apostrophe helps answer that question. Here's how to place an apostrophe in the right spot to show possession:
Tack an apostrophe to a plural noun that ends in the letter s to create a possessive.
Add an apostrophe and the letter s to a plural noun that doesn't end in the letter s.
Possessive pronouns (my, his, theirs, whose, and so forth) never contain apostrophes.
Place a possessive noun or pronoun in front of an -ing verb form that is being used as a noun (my swimming, Arthur's dancing, and the like).