Many writers believe that commas are necessary in the middle of a sentence only when they divide two independent clauses — two parts of a sentence that could stand on their own as distinct statements. But commas also serve as comprehension aids: Note the difference between “Knights wore metal shoes and gloves called gauntlets” and “Knights wore metal shoes, and gloves called gauntlets.” The first version implies that both items were referred to as gauntlets, and the second version correctly distinguishes that only the second item was labeled as such.
“Are they going to lock me up or shoot me?” looks like the writer is asking if one of these two outcomes will occur. “Are they going to lock me up, or shoot me?” correctly clarifies that the writer is asking which outcome will occur — and that’s a big difference made clear by the mighty little comma.
Traditionally, a comma was inserted after all introductory phrases, no matter how short: But the trend toward open punctuation and away from closed punctuation has relaxed this tradition. Unfortunately, though short introductory phrases may not look wrong in isolation, in text containing both short and long introductory phrases, when the latter cry out for a comma to give the reader a rest, inconsistency is awkward, so it’s best to always retain closed punctuation.
Sometimes, misunderstandings may occur when you omit a comma, as when a reader reads, “When she returned Jim’s head was already lying back against the pillow” and thinks at first that Jim’s head is being handed back to him — or its current owner.
Comments actually or conjecturally directed toward readers or a third party are awkward without a comma following an imperative (a form of address that tells someone to do something). The warning statement “Move over RCA and Sony, computer firms are becoming TV makers” starts the reader off at a disadvantage; who, they may think, is steamrolling over the television manufacturers in question? A comma after “move over” solves that problem.
Similarly, “Attention shoppers!” implies that attention is a commodity some store patrons are there to buy, and that the voice on the intercom is acoustically accosting just that class of consumer; “Attention, shoppers!” meanwhile, asks for something, then identifies who is being asked.
These rules may seem complicated. But there’s a simple test that usually works: When in doubt about whether or where to place a comma, read aloud the sentence in question, and visualize the comma as a hook that briefly makes the sentence run in place. If you hesitate or pause, insert the hook in the sentence to mark that place. If you don’t, don’t.