Commas are a way of separating sentence content from the rest of the sentence. You may not separate a subject from its verb. You can do other things with a single comma, though. As single comma, usually after the first word in the sentence, can be direct address. That’s when you name the person or thing you are speaking to. (Charlie, get out of bed!) It can also separate something parenthetical, such as a conditional clause. (If you don’t get out of bed now, you’re going to miss the bus!) A single comma can also separate something called an appositive. An appositive is renaming something; it’s equivalent to an equals sign. Here’s an example of that from a recent Bizarro comic. Read the apron. I confess I’m not much into rock and roll, so I just barely know that “Kiss” is the guy’s name. Or something.
What about two commas? The rule in writing is that you don’t separate a subject from its verb with a comma. But you may use two commas. Two commas enclose a parenthetical remark. Since it’s parenthetical, it doesn’t count as part of the sentence. Let’s modify the above:
Kiss, the cook, sports a rather unconventional appearance.
You can take out “the cook” and you still have the main sentence. Do not say, “Kiss, the cook looks rather unconventional,” unless you’re talking to Mr. Kiss about a cook.
A little more about parenthetical remarks: You can make them three ways. I already mentioned commas. Use commas for a minor aside. Use parentheses (which I use rather often in my writing) for remarks that are somewhat off topic. Finally, use M-dashes—very handy to know how to use—to emphasize the importance of the remark. You make a M-dash by holding down the Alt key while you type 0151 on the numeric keypad. Mac users, you’re on your own, and some word processors have their own way of making them. I’ll belabor the point:
Kiss (did you know he can cook?) is pretty good with a barbecue grill. Kiss—he is actually a very good cook—served up some excellent spare ribs.
A final parenthetical remark: You really should kiss the cook.