If you want to show ownership in the case of a singular noun, add an apostrophe and an S.
Examples: "the tiger's stripes," "the airplane's wings," "the boy's book"
If you have a series of nouns and the ownership is shared, add just one apostrophe and an S to the last noun in the series.
Examples: "Tim, Janet and Bill's house," "McDonald and Smith's book on sailing"
But if the ownership is separate, add an apostrophe and an S to each noun in the series.
Example: "Julie's, Ben's and Kate's coats"
To show ownership in the case of a plural noun ending in S, just add an apostrophe.
Examples: "the girls' team," "the guests' luggage"
Some plural nouns are irregular and don't end in S. In this case, add an apostrophe and an S.
Examples: "children's shoes," "the media's influence," "the men's restroom"
Proper nouns work the same way as regular nouns. If the proper noun is singular, add an apostrophe and an S to show ownership.
Examples: "Tim's boat", "Italy's wine district," "Barcelona's nightlife"
If the proper noun is plural, just add an apostrophe.
Examples: "the Alps' glaciers," "the Smiths' house"
Don't use apostrophes in numbers to show centuries or decades. Just add an S to form the plural.
Examples: "the 1600s, the 1930s"
Sometimes we might abbreviate a number to show a decade. In this case, use an apostrophe in place of the missing numerals.
Examples: "the music of the '70s," "the Roaring '20s"
Don't use an apostrophe to make an abbreviation plural. Just add an S.
Examples: DVDs, IRAs, PhDs
In contractions, use an apostrophe in place of omitted letters, like in I'll, won't or couldn't
Be especially careful with pronouns. If you're using an apostrophe with a pronoun, make sure that the meaning calls for a contraction. For example, "it's" is short for "it is." The possessive form of it is "its" without the apostrophe.
Examples: "It's time for lunch." "The dog chased its tail."
Who's and whose are another pair often confused. In this case, remember that the apostrophe stands in place of the missing letters. "Who's" is short for "who is" or "who has." The possessive form is "whose," without an apostrophe.
Examples: "Who's that knocking at the door?" "Whose book is this?"
Generally, apostrophes are not used to show a plural form. But there is one exception to this rule. You can use an apostrophe to pluralize letters of the alphabet, as in: "Mind your P's and Q's."
If a proper noun ends in S (like "Dickens" for example) and you want to show ownership, this is where things get a little confusing, because the writing guides don't all agree on what do to. The MLA Handbook says to add an apostrophe and an S, as with any other singular noun.
Examples: "Dickens's novels," "Venus's orbit"
The Associated Press Stylebook says to use just an apostrophe.
Examples: "Dickens' novels," "Venus' orbit"
In a case like this, consider your audience. Are you writing a research paper for a school or college course? Then ask your instructor which style guidelines to use. (Most schools and universities prefer the MLA guidelines.) If you are writing an article for a newspaper or other non-academic publication, you'll probably want to use the Associated Press guidelines. But check with your editor first.