Use an apostrophe to form possessive nouns. A possessive noun shows ownership. Add 's to singular nouns to show ownership, such as "the girl's dress," "the book's cover" or the "student's name." Add 's to plural nouns, such as children and moose, that do not end in an s, such as "the children's clothes." Use 's with compound words, such as "my sister-in-law's house" or to show joint possession such as "John and Mary's car." Do not use an s for plural nouns that already end in "s." Use only an apostrophe, such as "the boys' mothers" or "the girls' books."
Use an apostrophe to show an omission of letters in contractions, such as "I'm" for "I am" or
"don't" for "do not". This also includes years, such as '80 for "1980."
Form the plural of lowercase letters with an apostrophe, such as "b's", "c's" or even "abc's". Uppercase letters do not require an apostrophe.