Rule 1. Never use an apostrophe to make a word plural. Never! No exceptions. Not even if the word ends in a vowel. The plural of apostrophe is apostrophes. The plural of 1920 is 1920s. The plural of LCD is LCDs. The plural of do is dos. Really.
Corollary: Do use an apostrophe to make a single letter of the alphabet plural. Example: There are two p's, four i's, and four s's in Mississippi.
No to words, yes to letters.
Rule 2. Use an apostrophe to replace the missing letter(s) in contractions. Most of us don't have too much trouble with this one. We know how to spell words like I'll, he'd, can't, shouldn't, it's (meaning it is).
Put Rule 1 with Rule 2, and you become one of the few people in the United States who know how to spell dos and don'ts.
Rule 3. Use an apostrophe to make a noun possessive. Put it immediately after the noun that tells who is doing the possessing. (If you're not sure, just turn the phrase around and you'll instantly see where the apostrophe goes.) Examples:
- the boy's dog = the dog that belongs to the boy
- the boys' dog = the dog that belongs to the boys
- the Smiths' house = the house that belongs to the Smiths
- the smith's house = the house that belongs to the smith (would he be a blacksmith, perhaps?)
- the woman's room = the room belonging to the woman
- the women's room = the room intended for women (since the word womens doesn't exist, there is no reason to put the apostrophe anywhere else)
- the newspaper's reputation = the reputation of the newspaper
- the newspapers' reputation = the reputation of the newspapers
- his dog = the dog that belongs to him
- their house = the house that belongs to them
- her room = the room that belongs to her
- its reputation = the reputation that belongs to it
OK, you're an apostrophe expert now. How hard was that?
If you want to debate the fine points, you can always discuss what to do with words that end in s or z. Is it Jesus' name or Jesus's name? Thomas' car or Thomas's car? Style books differ. But notice that the apostrophe still follows Rule 3 above: it still comes immediately after the noun that describes who owns the name (Jesus) or the car (Thomas).