1. Use an apostrophe to form the possessive.
Add -‘s to nouns not ending in -s. Singular nouns which do end in -s may take either an ‘s or just an apostrophe.
Example: That is Nathan’s briefcase, not James’s [or James’].
Add only an apostrophe to plural nouns which end in -s.
Example: The articles’ titles were similar.
2. Do not use an apostrophe to form possessive pronouns:
its, whose, his, hers, ours, yours, and theirs.
3. -Ing verbs used as nouns or in noun phrases (called gerunds or gerund phrases) sometimes require possessive forms.
Examples: Mary’s writing is honest and full of detail. (Mary owns her writing.)
One feature of his upbringing was his parents’ constant arguing over money. (The parents own their arguing.)
4. Remember that some possessives don’t feel possessive.
Example: At day’s end… (Does the day “own” its end? That’s somewhat hard to imagine. The ‘s means of the, so we have “at the end of the day.”)
5. Use an apostrophe to form contractions.
Remember, the apostrophe stands for the letter[s] left out.
6. Use an apostrophe to form plurals of numbers, letters, words, and abbreviations.
Example: You’ve used seventeen very’s in one page of writing.