An apostrophe can be used to indicate the possessive form of a word or to build a contraction.
Step 1: Make singular nouns possessive Add an apostrophe and an s to the end of singular nouns, such as "book," and to indefinite pronouns, such as "another," to make them possessive. Do this even if the word ends in s.
TIP: Add only an apostrophe to the end of ancient proper names ending in "es" or "is" -- like Achilles’ heel or Isis’ temple - or any other noun where the extra syllable of the apostrophe "s" sounds awkward, to make them possessive.
Step 2: Form the possessive plural of nouns Make plural nouns ending in s, such as "cars," possessive by adding only an apostrophe. For irregular plurals that don't end in s, such as fish, add an apostrophe and s to the end.
Step 3: Add apostrophe and s to compound nouns Place an apostrophe and an s on the end of compound words, such as "father-in-law," to make them possessive.
Step 4: Indicate ownership Indicate ownership for two or more items by adding an apostrophe and s. For separate ownership, add an apostrophe and an s to each noun; for joint ownership, add an apostrophe and an s to the last noun.
Step 5: Form contractions Form contractions to combine two words by using an apostrophe where the omitted letters would go. For example, change do not to don't.
TIP: Use an apostrophe as a contraction to represent the century when writing the year in informal writing. For example, class of '03.
Step 6: Form plurals of letters Use an apostrophe and s in a few rare cases to form plurals of lowercase letters, such as in the phrase "p's and q's."
Step 7: Avoid common errors Avoid the common error of using apostrophes for personal pronouns such as "its," or plural nouns that are not possessive.
FACT: The word apostrophe has its origins in Greek, and means "to turn away from."