Let's say it's Hallowe'en, you're in Hawai'i, and you're doing t'ai chi while reciting the Qur'an. The obvious question is: What's up with all those apostrophes?
Seriously, why not just do tai chi in Hawaii on Halloween while reciting the Quran (or Koran)? In fact, many people would! Write it that way, that is.
Apostrophes, as we all know, seem to exist in English mainly to bedevil people. You'll see them in all sorts of places they shouldn't be, and not in some places they should ("Your not joking about those apostrophe's"). Our lives would probably be easier if we didn't use them. But they're not going away — people won't let them! Well, except when doing tai chi on Halloween in Hawaii. Many of us seem to be perfectly happy to get rid of those ones. Even though not all of them are actually apostrophes.
So why are they there in the first place? Why are Mr. O'Hara in Xi'an and Mr. Ohara in Xian two different people in two different places? It's because the apostrophe has become a universal widget to indicate things English speakers usually don't pronounce. Here are some reasons we press this tick-of-all-trades into service.
Because something's missing. This is the usual reason in English. It's why we borrowed the apostrophe from French in the first place, and some other languages use it that way too. A French printer named Geoffroy Tory is typically credited with inventing it in the early 1500s for that purpose, and we brought it over and started using it where'er we weren't quite saying a letter, whether it be a dropped vowel as in weren't or an elided consonant as in where'er — or Hallowe'en, which is from Hallow even. The even is the same one as in evening.
To stand for a glottal stop. This is an extension of the first one: If you drop a letter such as h or t when you're speaking, you may replace it with a glottal stop — just a momentary catch in the breath. Cockney speech is often represented this way: 'Ere! Wot's the ma''er? So when we're writing down words in another language that have a glottal stop, we typically use the apostrophe to represent it… except when we use that other thing that looks like an apostrophe but is not an apostrophe: the opening single quotation mark. It's used that way in several Polynesian languages, including Hawai'ian. That's right, you're supposed to say "Hawai'i" with a glottal stop between the i's, and you're supposed to write the "apostrophe" facing the other way. (Of course, if — like many websites, including this one — you use straight apostrophes instead of curly ones, they all look the same.)
For added fun, we have languages such as Hebrew and Arabic. Each of those has a character that can be a glottal stop or sometimes just not pronounced at all, and we usually transliterate it as an apostrophe — as in Qur'an — but each one also has another character that is a voiced pharyngeal fricative, a deep-back-throat sound unlike anything we say in English, and we typically transliterate that one with the other-way apostrophe. For example, Sana'a is a city in Yemen, and it has the pharyngeal sound. But actually, it would be more accurate to write it as San'a' because it has that glottal-stop character at the end. If you notice people generally writing it as Sana, now you know why: They couldn't be bothered with the marks, especially when English speakers don't even say them.
To stand in for an accent. Sometimes you'll see something like Niccolo' standing in for Niccolò just for ease of writing. Actually, you use an apostrophe to stand for an accent far more often than you probably realize: Every Irish family name starting with O' was originally a name starting with Ó. The O' doesn't stand for of; ó is a Gaelic word meaning "from" or, in this case, "of the family of."
To indicate palatalization. Palatalization is the difference between, for instance, the ll in billiards (which is palatalized: The tongue is pressed up at the front) and the ll in balloon (which is not). Words transliterated from a language such as Russian sometimes use the apostrophe to indicate it. You may see it in place names such as Podol'sk and Ural'sk — but then again, you may not: since English speakers don't make that distinction, we often leave it out.
To indicate an ejective. Some languages have ejective consonants — they're said with a kind of forced air pressure from the voice box. Perhaps the most famous language to use these isn't even real: Na'vi, the language from the movie Avatar. In Na'vi the apostrophe is used to indicate a glottal stop and x is used to indicate an ejective, but in many real languages — such as that of the Gwich'in people of northwestern North America — an apostrophe is used to indicate an ejective.
To indicate aspiration. Aspiration is like saying a little "h" after a letter. In English, we sometimes do it without thinking (like after the t in top but not the t in stop), but it doesn't make a difference in the meaning. In some other languages, it does make a difference. We often represent it with h when we're transliterating those languages — such as Thai. But occasionally we use… yes… the apostrophe. The Wade-Giles system of transliteration for Mandarin Chinese did this. It's why there's one in t'ai chi (or two in the full name, t'ai chi ch'uan). But that's no longer the standard way to write Mandarin Chinese in Roman letters. The official system is Pinyin, which spells t'ai chi as taiji. So in English, many people just throw up their hands — slowly, gracefully (this is t'ai chi, after all) — and write tai chi.
To prevent confusion. Pinyin still has a use for the apostrophe, though: to distinguish between, for instance, Xian (which is one syllable, said sort of like "shyan" — or "shan" if you're from the Great Lakes area) and Xi'an (which is two syllables, said sort of like "she an"). It gets used that way sometimes for other languages too.
The apostrophe also shows up in other places. If a language is written or transcribed in the Roman alphabet, and it needs a little symbol to indicate a little difference, the apostrophe is the ready all-purpose widget. For example, in Breton, ch spells a sound like "sh" while c'h spells a sound like the one you hear in German ach or Scottish loch.
But whatever apostrophes may indicate in another language, almost all of them have one thing in common when they show up in English: They don't make a difference in the way most of us say it — even if they should, as many people from Hawai'i will tell you.