Let’s start with the simplest apostrophe error first. Plural nouns do not have apostrophes, with the exception--and even this is in flux--of letters and numbers. Single letters, e.g. p’s and q’s, 6’s and 7’s, take the apostrophe, longer strings, e.g., 1960s, etc. do not. Correction to yesterday's post as it appeared: Maiya has reminded me that only single letters and numbers need apostrophes, which makes sense upon reflection. The plural of the letter ‘a’ without an apostrophe would look like the word ‘as,’ whereas ABCs is unambiguous as it stands. I still seem to remember numerical decades taking apostrophes in my younger days, e.g., 1950’s, but Maiya is correct that current usage (she says going back at least 30 years) is to omit the apostrophe, so write that decade as 1950s to avoid the copy editor's pencil. Just goes to show how we can keep each other on our toes. Thanks, Maiya!
But apostrophes can still be confusing, and people worry about leaving one out where they should have put one in, so the little squiggly superscript comma gets thrown into all kinds of situations where it doesn’t belong. When you see a sign for a “Farmer’s Market,” don’t go by the punctuation and expect to find one lone farmer. Similarly, it’s unlikely that an event sponsored by the “Lion’s Club” is being hosted by a single Lion. And when the sign says, “No Dog’s Allowed” or “No camera’s, please,” you can mentally delete those apostrophes, too. In each of these cases, a simple unadorned plural is what the writer intended. Several (possibly many) farmers, a group of lions, no (i.e., not one!) dogs or cameras!
Plural noun: no apostrophe (unless a single letter or number is standing in for a noun).
Possessive pronouns do not have apostrophes, either. My, our, your, his, her, their. Carry the lack of inappropriate decoration in these words over to his, hers, yours, theirs, its.
In short, just because a word ends with an ‘s’ doesn’t mean it needs an apostrophe!
Here’s where you do need it:
(1) Use an apostrophe when two words are joined in a contraction (do not = don’t; is not = isn’t, it is = it's, etc., including the first word in my sentence above, here + is = here’s);
(2) Use an apostrophe to show possession with a noun, common or proper (the tree’s leaves, the sun’s warmth, Sarah’s dish, Pamela’s bookstore, etc.).
In summary, yes, possession is sometimes indicated with an apostrophe and sometimes not, but the distinction is simple: possessive nouns take the apostrophe, possessive pronouns do not.
Then just keep in mind that the apostrophe in a contraction indicates a letter left out (such as the ‘i’ in ‘is’ or the ‘o’ in ‘not’).
There are a few special cases left out of this discussion, but for the sake of correcting 99% of common errors I’ve gone for the broad brushstrokes. Many years ago, I worked in a very boring office for a very dull boss. One day, he surprised me by saying something I found interesting. You may think this remark of his mind-numbingly dull, but I’m interested in language, and you have to know the kind of thing he usually said, word for word from day to the next. The unusual, interesting observation was: “We may see the disappearance of the apostrophe in our lifetime.” Little did that man anticipate that apostrophes would proliferate like spotted knapweed, popping up everywhere!
So don't be guilty! Nip those inappropriate apostrophes in the bud! Do not propagate! Your old high school English teacher will sleep better at night, knowing that you are doing your part.
Postscript: I've gone back into this post to add the most misunderstood pair of words related to this subject, its/it's. I hope you can see the difference! The cat drank all its milk. It's too late to give the kitten any. Possessive vs. contraction. See?
