Apostrophes have two primary uses:
Apostrophes are used to make contractions. When words are joined into abbreviated forms, apostrophes replace the missing letters.
Remember, contractions are not generally acceptable for formal writing.
Apostrophes are used to indicate ownership. In most cases, words will either receive an apostrophe, followed by an s, or simply an apostrophe.
The following situations receive an 's:
The following situations receive only an ':
When using pronouns, do NOT use an apostrophe to show ownership.