This is a really handy link that does a good job of explaining the basic (and not so basic) rules of comma usage. It even discusses some of the negotiable ones, like the so-called Oxford comma.
As an aside, commas seem to cause more problems for writers than all the other punctuation marks put together. I'm guessing this is because commas have more uses than all the punctuation marks put together. I don't know if grammar and punctuation are really getting worse overall, or if people have always been terrible, but the internet certainly makes it easy for people to display their ignorance. I generally try to not correct people on social media or internet forums, because there's usually a karmic comeuppance in store when one does this. It's always amusing to see someone taking another person to task for a specific grammar error when they've committed that same error (or a different one) in their own post.
It's pretty common for people to blame teachers for this sort of thing. I teach college biology, and anecdotally, I think your typical college (or even university) freshman has always been pretty bad at grammar and spelling. The majority had to take the infamous "English A" back when I was at UC Davis. I do think certain kinds of spelling and grammar errors are more common than they used to be, but I don't know that it's fair to say it's because "they don't teach this in school anymore." There has been a lot of grade inflation in the public schools (high school GPAs are, on average, higher than they were when I was young), and a higher percentage of the population attempts college, at least, than once did. We also have a lot of English learners in our community college student population. So even without the internet, these things will change the demographics, and create and apples and oranges dichotomy between the college students of yesteryear and today.
Plus, US society does not, in my opinion, value knowledge, or even the appearance of having knowledge, for its own sake. Ask the average person why they're in college, and they'll tell you, "To get a degree that lets me get a good job," not, "To become educated or become a better, more informed citizen and to develop skills that will allow me to embark on a lifetime of intellectual and professional growth and learning."
We like to laugh at people who "use big words," or "talk too smart."
But I digress.
I've been thinking about some of the things I run across fairly often that are serious pet peeves for me.
1. Misuse of apostrophes in plurals that aren't possessives or contractions. Just don't do this. Please. "Possessive's or contraction's." Whimper.
2. The possessive form of it with an apostrophe. It's=a contraction for "it is." Just as you don't use apostrophes for his or hers, you don't use them for its in this context. It's amazing how many professional web sites do this. We all make occasional typos, but when someone uses it's as a possessive throughout, it's pretty clear they never learned that rule back in school, and they haven't been observant in their daily life to pick up the correct usage.
3. Incorrect dialog tag punctuation and capitalization. When someone says something, you use a comma.
Bob said, "It's really very simple."
"It's really very simple," said Bob (I actually would prefer Bob said there, but "I did it this way to show how said shouldn't be capitalized here).
4. Using a word that really can't be "spoken" as a dialog tag instead of an action that occurs immediately before or after a spoken sentence.
"It's really very simple," Bob coughed.
"It's really very simple." Bob coughed.
5. Incorrect use of semicolons. Unless you are constructing "smileys" in a forum post or text message, these little puppies have two uses: separating two independent clauses within a sentence that lacks a coordinating conjunction (and, but, so etc.), or for separating the elements of a list when the list elements themselves contain commas.
Some people love dogs; other people fear them.
I have been to the beach in California, the Pacific Northwest, and Maine; to the mountains in California, Oregon, and Colorado; and to the desert in California, New Mexico, and Arizona.
Notice that the punctuation mark I used after "these little puppies have two uses" is a colon (:), not a semicolon.
6. Misusing the term "run on sentence" to generically refer to any long sentence. Run ons are, in particular, sentences that need a coordinating conjunction (with comma), or a semicolon, regardless of their length.
My dog hates my neighbor's dog they always fence fight.
This is a run on, because it needs a semicolon between "dog" and "they."
Or you could just write:
My dog hates my neighbor's dog, so they always fence fight.
It is possible to have a very long sentence that is not a run on. The issue is whether or not it contains independent clauses that are mashed together without appropriate punctuation.
7. Okay, this isn't a grammar or punctuation error, per se, but a spelling one. But here goes.
Loose and lose. They are two different words, people. They're not even homophones. This one seems to be proliferating like mold lately. I honestly don't remember getting e-mails or reading student papers that made this mistake until a few years ago, but now I see it all the time. Even on writers' forums. I really don't know what gives.
It is sometimes good for a giggle. When someone writes, "I'm always loosing my keychain," I imagine her tossing her keys up in the air and shouting, "Be free, little fob!"
8. Here's one that's a bit more obscure, maybe: fewer versus less. You use fewer for items that can be quantified exactly (as in counted), while you use less for grammatically single nouns described in relativistic quantities. For instance, you'd say, "There is less money in my wallet since I had kids," but, "There are fewer bills in my wallet since I've had kids."
I think this one is hard, because a similar distinction doesn't really exist for the analogous quantitative word "more." There is the "many" versus "much" distinction, however.
Okay, I've ranted a bit. Does anyone have "favorite" grammatical or punctuational peeves of their own?