Use the apostrophe to form the possessive of singular nouns and pronouns in combination with the letter "s." In this case add the apostrophe first and then the "s." Add apostrophe-s to the plural form of nouns or pronouns to create the possessive. Exceptions to this rule are:
When the noun itself is in plural form, such as "men" or "children," add apostrophe-s as with singular nouns.
Treat nouns that are the same in singular and plural form, such as "corps" and "deer," as you would the plural form of a noun: "the deer's tracks."
Pronominal pronouns (hers, his, its, ours, theirs and yours) do not use the apostrophe.
Indefinite pronouns use the apostrophe-s to form their possessive, as in "one's" and "else's."
To form the possessive of names, add apostrophe-s even when the name ends in an "s," such as "the Hendricks's house." Many times this can become cumbersome, so modern usage allows for the final s to be dropped, as in "the Hendricks' house."
Use the apostrophe to mark the omission of one or more letters. This can be in the form of a contraction such as "can't" for "cannot," "don't" for "do not" or "they're" for "they are." One of the worst misuses of the apostrophe is with the word "its." Use an apostrophe only to form the contraction it's (it is). Never use an apostrophe to form the possessive of "it." The proper spelling of its possessive is "its."
The apostrophe also marks the omission of figures in dates such as "summer of '69" or letters left out of truncated words such as "Jo'burg" (Johannesburg) or "J'ville" (Jacksonville).
An apostrophe is used to make the plural when it follows individual letters.
She got all A's in school.
You better mind your p's and q's