The apostrophe is the small raised mark in don't and Anna's. It does exactly two jobs in English, and almost every apostrophe mistake comes from asking it to do a third.
Job one: letters left out
In a contraction, the apostrophe stands where letters were dropped:
- do not becomes don't, is not becomes isn't
- it is or it has becomes it's
- they are becomes they're, you are becomes you're
- I would or I had becomes I'd
The same idea covers shortened years (the summer of '69) and a few fixed words such as o'clock. Contractions are normal in everyday writing; very formal documents often spell the words out instead.
Job two: possession
The apostrophe also shows that something belongs to someone:
- Singular nouns add 's: the cat's bowl, my sister's car.
- Plural nouns ending in s add only the apostrophe: the cats' bowls (several cats), my parents' house.
- Plural nouns that do not end in s add 's: the children's toys, the women's team.
- Names ending in s can take either form: James's book or James' book. Style guides differ (The Chicago Manual of Style prefers James's); choose one and stay with it.
- Two owners, one thing: only the last name takes the apostrophe: Anna and Tom's flat. Two owners, separate things: both do: Anna's and Tom's phones.
What an apostrophe never does
It never makes a plural
Apple's for sale and DVD's are wrong: the plurals are apples and DVDs. Decades work the same way: the 1990s, not the 1990's. The one widely accepted exception is a lower-case letter, where the apostrophe stops a misreading: mind your p's and q's.
It never goes in a possessive pronoun
Possessive pronouns already show possession, so they take no apostrophe: its, whose, yours, hers, ours, theirs. This is the source of the most common apostrophe mistake in English:
- The company changed its logo. Possessive, no apostrophe.
- It's a new logo. Short for it is.
If you can replace the word with it is or it has, write it's. If not, write its. The same test sorts out who's and whose, and you're and your; more in What Is a Homophone?
Straight or curly?
Typed on a keyboard, the apostrophe is a straight mark ('). Typeset text usually uses the curly form (’), which word processors insert automatically. Both are correct, but mixing them in one document looks untidy. One trap: at the start of a word, as in 'til or '90s, automatic quotes often insert an opening quotation mark (‘) facing the wrong way.
How the spell checker helps
A contraction typed without its apostrophe, such as dont or isnt, is not a word in the dictionary, so the Spell Checker flags it as a spelling mistake and suggests don't or isn't. The harder cases are pairs where both spellings are real words, like its and it's, or the dogs and the dog's. Those depend on the sentence, and the Spell Checker's grammar rules catch many of the common ones. The rest are worth a careful read, using the tests above.