When do I use an apostrophe?

• Use an apostrophe for contractions. Examples:
  • "don't" is a contraction of "do not"
  • "it's" is a contraction of "it is"
  • "let's" is a contraction of "let us"
• Use an apostrophe for possessives of nouns, but not pronouns. Examples:
  • "John's thing" means "thing" belongs to "John"
  • "its thing" (no apostrophe because "it" is a pronoun) means "thing" belongs to "it"