I'm going to be on the road starting tomorrow, and unlikely to have the time or inclination to post until after Christmas. I thought today could be another grammar gripe, to carry you through the week.
Commas are a useful piece of punctuation. I use them daily, if not hourly. When used properly, they aid in the flow of sentences and make for a seamless reading experience. They clarify (see Lynn Truss's Eats, Shoots & Leaves for many examples on how), they separate, and allow for parenthetical asides without the intrusion of parentheses. The oxford comma is hotly contested, and if you read the previous sentence carefully you'll see where I stand on that argument.
What commas do not do, however, is separate sentences. You cannot take two whole sentences and splice them together with a comma. Sometimes a semicolon is necessary, if the two sentences are closely related and interdependent, though I'm told they're discouraged by most modern publishers.
Those are, logically enough, called comma splices, and they sound, in my head, like the writer is out of breath. While writing is difficult, it's rarely strenuous, and so I regard such passages warily.
A sentence is composed of a subject and a verb. Rarely, though, are sentences mere subject and verb. They're often fluffed up with adjectives, adverbs, prepositions and conjunctions. I can't say a sentence with more than one verb is a comma splice, because I have several examples right in this post where they're not. All of those verbs are separated out into their own separate pieces of the sentence, but it's difficult to articulate how without teaching you how to diagram sentences.
Incidentally, I learned how to do that in middle school, and I hated it. But it did teach me about sentence structure and grammar rules and a grasp of the English language that I didn't previously have, so I'm grateful for the experience.
So, without all the strain and stress of diagramming your sentences, how do you learn to avoid comma splices? By reading your sentences carefully. If your thought is finished but the sentence is not, put a period. If you really, really need to link two sentences, put a semicolon. Ask what your commas are linking. If it's adjectives, you're good. If it's prepositions or lists, that's fine. It's when you're linking thoughts that belong in different sentences that you have a problem.
Have a very happy holiday, no matter what you celebrate, and I'll see you all before New Year's to post a wrap-up post or two. Meanwhile, feel free to request blog topics or share your thoughts in the comments.