COMMA USAGE MADE SIMPLE Copyright 2005, Michael LaRoccaDon't they drive you nuts?
You can visit all rules of style you want, and you can read all books and articles you want. You'll still be confused. You'll see inconsistency. You'll see experts who don't agree with each other. And, you'll pull out your hair. Unless you're me, since my hair's falling out all by itself. I think it'd do that even if I weren't an editor hunting down errant commas.
Well, folks, here are some rules. A bare minimum. Internalize these and ignore everybody else.
(1) Never put a comma between a subject and a verb. It's always wrong. The dog, barked. What is that? Idiocy. Read it aloud, and pause at comma. Don't you feel stupid?
(2) If you want to separate a clause, put a comma on both sides of it. Otherwise, no commas at all. "The dog, who held a bone in his mouth, ran to porch." See how there's a comma on both sides? That's because you could skip that whole clause entirely and it'd still be a complete sentence. "The dog ran to porch."
If I delete first comma, I have to delete second one. You decide which looks best, two commas or none. But, one comma doesn't work. Try deleting either one and reading result aloud, remembering to pause at comma. It's a wreck, isn't it? You don't talk like that, so don't write like that.
(3) "He saw cat, cat was on couch." This is not a good sentence. It's two sentences. The one before comma has subject/verb/object, and so does one after comma.
Run-ons like that can emphasize run-on nature of a character's words or thoughts, but use device sparingly. It's okay to break a rule, as long as you know what it is and why you're breaking it.
But in example above, it'd be best to make them two sentences. If you find you just can't do it, consider a semicolon. Don't believe anyone who says semicolons aren't allowed in fiction. I wouldn't use one in sample sentence, but I've used them in other sentences I've written. Sparingly.
But for something as lame as a sentence about a cat on a couch, it's best to follow rules exactly and make that two sentences. Do you really think your reader's gonna pop off for a beer or a toilet break between them and lose his place? As long as they're in same paragraph, they'll be read together.