Use an apostrophe before an s ('s) if the noun is singular and doesn't end with the letter s. Examples include: Bob's car, one nurse's uniform, New York's subway, a fish's fin, a woman's dress.
Use an apostrophe only, at the end of the word, if the noun is plural and ends in an s. Examples: several nurses' uniforms, sisters' recipes, lions' manes.
Use an apostrophe ('s) if the noun is plural and doesn't end with the letter s. Examples are all women's purses, the goose's feathers, children's teeth.
Use an apostrophe ('s) if the noun is singular and ends with the letter s, ss, or an s sound. Examples: my boss's shirt, the bus's passengers, Lois's money, Phoenix's airport.
Use an apostrophe ('s) for joint possession. Apostrophe placement depends on whether the nouns are acting together or separately. Example of nouns acting (or possessing) together: Kim, Daniel and Zoe's favorite ice cream is vanilla. Example of nouns acting (or possessing) separately: John's and Bob's ideas are different.
Use an apostrophe -s ('s) on the last word of a compound noun. Example: mother-in-law's hat.
Contractions: Use one apostrophe in place of the letter or letters omitted when one or more words are shortened. Examples: it's (it is), can't (cannot), they're (they are), ma'am (madam), o'clock (of the clock).