1. If singular noun, add ’s to make it possessive
Mistake: It was Johns’ turn to take the kids to school.
2. Possessive forms of pronouns don’t take apostrophes because they are already possessive.
Mistake: The boat is their’s.
*The possessive form of who is “whose.” Who’s=“who is.”
*The possessive form of it is “its.” It’s=it is.
Mistake: The man, who’s name is Mike, will donate.
Mistake: Give the cat it’s toy.
3. To make a plural noun possessive, add an apostrophe.
Mistake: The horses stalls were filthy.
*If the plural noun doesn’t end in s, add ’s.
Mistake: The peoples’ decision was to recall the governor.
4. An apostrophe can also indicate removal of letters.
Don’t, Isn’t, Didn’t: the “o” in “not” has been removed.