Written by furious kitten
There are two ways that apostrophes (‘) are used in English. The first is to show possession, and the second is to show when letters are missing, as in contractions.
Apostrophes are added to names or other nouns along with an s to show possession.
Ex.: Edward thought Bella’s truck was unsafe.
Ex.: The truck’s frame was virtually indestructible.
Possessive pronouns (mine, yours, his, hers, theirs, ours) are already possessive and do not require an apostrophe.
Ex.: Having known Bella all his life, Jake could easily guess which car was hers.
When making a word that ends in s possessive, you can either add just an apostrophe or an apostrophe and an extra s. Both are correct, and the difference is merely a style preference.
Ex. James’ smirk made Bella’s blood run cold.
Ex.: James’s smirk made Bella’s blood run cold.
One exception to this is if your noun is plural. Then you only add the apostrophe, and it’s not okay to add a second s.
Ex.: Bella found she did care about her friends’ futures.
To simplify, I would recommend always leaving off the second s. This way you’re doing the same thing whether your noun is singular or plural.
Apostrophes are used to show when letters have been removed in contractions. For example, in don’t, the o from do not has been deleted and replaced with an apostrophe. The apostrophe can also replace more than one letter, like the ha in should’ve from should have.
Ex.: Bella was beginning to think she should’ve agreed to marry Edward earlier.
Ex.: “Let’s head down to First Beach, Jake,” said Bella.
Its versus It’s and Other Common Mixups
Its is used to show possession, while it’s is the contraction for it is.
Ex.: Edward showed Bella that the quickest and cleanest way to eat a mountain lion was to immediately break its neck.
Ex.: “I think it’s raining out today,” Alice said.
Were versus we’re
Were is the past tense of are. We’re is the contraction for we are.
Ex.: When Edward chartered a boat from Rio, Bella wondered if they were heading toward Africa.
Ex.: We’re going hunting, Alice, would you like to come?
Also see Commonly Confused Words.
When NOT to Use Apostrophes
Apostrophes should not be used to make a noun plural. Only an s is added in that case. For example, house becomes houses, NOT house’s.
Ex.: Bella started to contemplate how many houses Edward’s family owned, then thought better of it.
This holds true for family names. When referring to the entire Cullen family, they are the Cullens, NOT the Cullen’s. This is tricky because your word processor will probably try to correct it, but stick to your guns on this one.
Ex.: Charlie liked all the Cullens, but he especially had a soft spot for Alice.
A Note About the Origin of Possessive Apostrophes
This isn’t really necessary to know to be able to use apostrophes, but as a grammar nerd I think it’s interesting. While the two uses of apostrophes I discussed above (possessive and contractions) may seem unrelated, they’re actually not. Possessive apostrophes arose out of contractions. In Old English, it was common to add es to the end of a noun to make it possessive. This was obviously confusing on words like hero, which already took es to become plural (heroes). Somewhere along the way, people began dropping the e from possessive words to show a difference between plural and possessive, and they used an apostrophe to show there was a missing letter (as with the contractions above). Heroes became hero’s. Eventually English-speakers forgot about the extra e and just learned about two different uses for apostrophes. Source: Grammar Girl.