The comma is the least emphatic, most humble, and yet most utilitarian of punctuation marks. Nevertheless, the poor comma is governed by so many rules that we despair of remembering them all. Garner's Modern American Usage identifies nine uses for the comma, most of which are rooted in logic and common sense.
As everyone knows, the comma indicates a pause and helps the reader avoid misreadings or misunderstandings in text.
Example: When I bake my dog begs for scraps.
Better: When I bake, my dog begs for scraps.
The comma makes the intent of the author clear, and the reader moves along without distraction. If you're not sure whether to use a comma, read the sentence aloud. Insert a comma where you would pause naturally.
Serial Comma (also
known as the Oxford or Harvard Comma)
Does the following sentence need a comma?
Commas are used before conjunctions (and, but, or, nor, for, so, yet) that join two complete sentences (independent clauses). But the verbal phrases "spoke to the jury" and "was ineffective" both apply to the same subject and, consequently, should not be separated by a comma. You wouldn't use a comma in the sentence "Robert spoke and sang" for the same reason. However, you would need a comma in "Robert spoke, and then he sang," because you have two complete sentences. There are so many rules of grammar that only an expert can come close to knowing all of them.
Dates, Addresses, and Professional Titles
Use commas with dates to separate the day
from the year - e.g., April 1, 2004. If the day is omitted, then omit the
comma (April 2004). If the date is inverted (1 April 2004), then no commas
Separate a title following a name with a pair of commas.
Beware the comma splice (also known as a comma fault, run-on sentence, comma blunder, or comma error). This grammatical nightmare is defined as two independent clauses that have been joined incorrectly.
The Longman Guide to English Usage (1988) notes that "experienced writers sometimes use commas alone for literary effect in linking a series of (often three) parallel sentences; a well-known example is I came, I saw, I conquered." Many authors are guilty of using comma splices (e.g., John Updike, E.B. White, William Faulkner) for literary effect. But in formal writing, the comma splice is the mark of a careless writer. This kind of error in formal writing can generate uncomplimentary remarks. Moral: Check your writing.
Restrictive and Nonrestrictive Phrases
Use commas to mark the beginning and end of a parenthetical word or phrase.
Do not omit the second comma unless a semicolon or period ends the thought for you. The missing comma prompted editor Lynne Truss (Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation) to comment: "Sensitive people trained to listen for the second comma find themselves quite stranded by that kind of thing. They feel cheated and giddy. In very bad cases, they fall over."
Terms of Direct Address, Interjections, Yes and No
Use commas to set off nouns of direct address, the responses yes and no, and interjections.
The general rule of thumb is to use commas to separate numbers with more than four digits. If the number is four digits long, a comma is optional.
Many sources correctly observe that the modern author uses fewer punctuation marks than the grammar books suggest. Yes and no. Informal writing is writing for effect. One-word sentences, bizarre capitalization, and oddball punctuation can all be used with positive effect. YIKES!!! But business writing and formal writing are both more impressive when the rules are followed. These are situations when you are trying to communicate facts, opinions, and ideas, not emotions. If you are a formal writer, then you must follow the rules 99 times out of 100. It is little wonder that grammar gurus take pride in their knowledge. It is one of the keys to the kingdom.