I got the following response: "Can you do a post on this? I need to learn about possessives'. (I did that apostrophe on purpose, Professor!)"
After arming myself with Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation, by Lynne Truss, and The Elements of Style by William Strunk and E. B. White, 2005 edition, I should be able to write this without any serious errors. But if not, any errors are mine.
Possessives in English are, for the most part, straightforward, at least for most singular nouns. No learning intricate rules for noun declensions. Just put an apostrophe and an "s" after a singular noun, and you've made it possessive. Stephen's blog, Kellie's column, Alaska's governor, the philosopher's stone. But English has exceptions--lots of exceptions--that confuse a lot of people.
The rule works fine until you get a singular word ending in a sibilant. And most of the time it still applies: Kellie Davis's blog, Charles Dickens's novels, the octopus's garden, the moose's antlers. But if you get too many sibilants, the rules of style call for an apostrophe at the end of the word: Massachusettts' governor, or for convenience' sake. Jesus and Moses usually merited exceptions: Jesus' parables; Moses' laws. But I've seen Jesus's a lot lately. And if you get a word ending with a silent "s" (usually a French borrowing), put the apostrophe at the end: Illinois' ex-governor, Des Moines' museums, Arkansas' Huckabee. That way you just say the one "s," and don't say anything barbarous, like Illinoise's. As for the Illinois city of Des Plaines, where they pronounce the final esses, I'd hold with the final apostrophe. Strunk and White suggest another way of dealing with too many sibilants: say "temple of Isis" instead of Isis' temple.
And then there are plurals. Thanks to William the Conqueror, better known to his contemporaries as William the Bastard, and not just for his parentage, English acquired a lot of French grammar and vocabulary. On top of the Germanic "s" for possessives, we also have the French "s" for plurals. What to do? For regular plurals ending in "s," just put the apostrophe at the end of the word: the Davises' house, the Wylders' folly, the dogs' breakfasts, etc. Irregular plurals not ending in "s" go back to the rule for singular nouns: men's room green and women's room pink.
With compound nouns, you apply the apostrophe and "s" to the last word: My son-in-law's Facebook page says he's Lt. Worf. I'm not sure about plurals, here: sons-in law, attorneys general. I'd avoid the possessive case and use a preposition: the meeting of the attorneys general.
If you need to show joint possession, the apostrophe goes with the last in the series: Steve and Kathleen's house, Bob & Carol & Ted & Alice's bed, etc.
But it's the pronouns and determiners--especially "its"--that give English speakers the most problems. Our case system, unlike those of Latin, German, or the Slavic languages, is based on word order, prepositions, and, in the possessive, or genitive case, the letter "s" and an apostrophe. No tedious noun or article declensions. Except for pronouns and determiners. Lynne Truss conveniently provides a list of possessive pronouns and determiners:
Possessive Pronouns: mine, yours, his, hers, its, ours, yours, theirs.
Possessive Determiners: my, your, his, her, its, our, your, their.
They're part of our Anglo-Saxon heritage, and they don't take an apostrophe. It's just that the word "it's," a contraction of "it is" or "it has," is pronounced the same, and has an apostrophe, though it's serving an entirely different purpose: to mark it as a contraction at the point where we've removed the letters. (Very traditional writers describe October 31st as "Hallowe'en" to remind us that it's All Hallows' Even.) Example: Even though the tree has lost its leaves, Kathleen knows it's a hackberry from its bark.
I wish there were a mnemonic for this, such as the one my generation learned for the order of planets: My very educated mother just served us nine pickles. (Doesn't work now, because Pluto's been stricken from the list of planets.) But I can't find one for its and it's. Once more:
"It's" is a contraction of "it is" or "it has." "Its" is the possessive.
I've noticed some confusion along the same lines between "your" (possessive) and "you're" (contraction of "you are.") and between "their" (possessive) and "they're (contraction of "they are.").
You're never going to finish your novel if you keep wasting time on Facebook.
They're painting the passports brown. People with brown passports should apply for their visas at 1313 Desolation Row, just off Highway 61.
One more thing: Purdue University has a wonderful site on writing called the OWL. Check it out here. The page on apostrophes is here.
There you go, Kellie. Sorry so many of my examples date back to the 1960s (note: no apostrophe there-it's plural, not possessive). It's because I date back to the '60s (apostrophe there because I've left out the "19"). Now it's back to trackside.