Things you ought to know about apostrophes:
Don’t use them to form plurals… EXCEPT when not using an apostrophe would make the plural look like something else. The plural of B — such as when you are discussing the grades someone earned at school — is Bs, but the plural of A is A’s, because otherwise it would look like the word As. So, y’know, remember to cross your Ts and dot your I‘s. 🙂 (I have no idea why WordPress’ sorry-ass excuse for a spell check program doesn’t like the correct plural of B, by the way, but what can you expect from a program that, just a few days ago, declared that commas isn’t a word?)
Apostrophes indicate missing letters. That’s why we use them to form contractions: don’t is the shortened form of do not. (Yes, I know Microsoft Word doesn’t approve of contractions. Fuck Microsoft Word and its green squiggly lines. Contractions are less formal, but they are not grammatically incorrect.) Apostrophes aren’t just for contractions, though. They’re also used to show that a letter has been dropped from the beginning or end of a word, as with the famous trio of readin’, writin’, and ‘rithmatic. It is possible to go overboard with this (as every person who has ever read The Adventures of Huckleberry Finn knows — ohmigod, the apostrophes spattering the pages like freckles on a redhead), but that’s because it is possible to go overboard with writing in dialect anyway; the punctuation itself is not at fault.
And while I’m on this topic… The correct shortened form of because is NOT cos; it’s ’cause. Think about this, folks. The word because doesn’t even contain an o — where’d that come from? The s is pronounced as a sort of soft z anyway; even spelling it coz would make more sense (but still be wrong).
Use apostrophes to indication possession. (*pauses for jokes, ’cause you’re such creative people that of course you’ll see the potential for twisted humor in that sentence*) Once upon a time, if a book belonged to some guy named John, it would be referred to as John, his book, which later became shortened to John’s book. Just in case you’re wondering where the apostrophe-s thing came from… Mind you, there is some debate about whether or not to add an s after the apostrophe when the noun itself ends in s. I think the extra s is silly and leads to weird pronunciations. (For whatever reason, some people will add an extra s or several anyway: We buys our groceries at Krogersesss, my Preciousss, says Gollum. 🙂 ) However, it is more accepted to write or Santa Claus’s sleigh rather than Santa Claus’ sleigh.
Occasionally, apostrophes come up in discussions about writing fantasy or science fiction; the common “wisdom” is that names containing apostrophes are a certain sign of Bad and Amateurish Writing, because inexperienced authors will use them to make names “different”: Apostrophes make you alien, as one version of the snark goes. Hmm… So what all those people are saying is that real humans never have Irish surnames such as, for example, O’Neil (or O’Neill — preferred spelling may depend on whether you’re more a fan of Teenage Mutant Ninja Turtles or of Stargate: SG-1); real humans aren’t from Hawaii (which is, for the record, also spelled Hawai’i). And the magazine editor who rejected the first short story I ever tried to get published definitely wasn’t a real human: he had not one but two apostrophes in his name. So as far as insistence on that “rule” concerning apostrophe usage goes… Get over your English-centric worldview and use the brains evolution gave you. Please.