From a David Foster Wallace Fall 2002 class handout now online, Your Liberal-Arts $ at Work:
For a compound sentence to require a comma plus a conjunction, both its constituent clauses must be independent. An independent clause (a) has both a subject and a main verb, and (b) expresses a complete thought. In a sentence like “He ate all the food, and went back for more,” you don’t need both the comma and the and because the second clause isn’t independent.One mistake: the sentence “An independent clause (a) has both a subject and a main verb, and (b) expresses a complete thought” should not have a comma: it has only one clause.
A second mistake: “He ate all the food and went back for more” is a single independent clause, not two clauses. Notice that the sentence explaining an independent clause and the sample sentence follow the same pattern: subject-verb-and-verb. Neither sentence needs a comma.
But there’s more. Look carefully at the third sentence:
In a sentence like “He ate all the food, and went back for more,” you don’t need both the comma and the and because the second clause isn’t independent.That sentence needs a comma before because, for the very reason that Wallace explains later in the handout:
[B]ecause is a funny word, and sometimes you’ll need a comma before its appearance in the second clause in order to keep your sentence from giving the wrong impression.Look again:
In a sentence like “He ate all the food, and went back for more,” you don’t need both the comma and the and because the second clause isn’t independent.The first version misleads by suggesting that you don’t need both the comma and and for some other reason.
In a sentence like “He ate all the food, and went back for more,” you don’t need both the comma and the and [,] because the second clause isn’t independent.
There’s a fourth mistake in passing: afterwards and backwards are not prepositions. And I suspect that Wallace’s observations about a sentence being “nonstandard in the abstract” would set linguists howling.
Pedantry is always tiresome, but it’s especially tiresome when the pedant doesn’t know what he is talking about. I’m reminded of the poet Ted Berrigan’s comment about another Dave, a friend:
“Dave knows just enough to get himself in trouble. . . . He says her name is pronounced Gertrude SCHTEIN because that’s the way German is pronounced. He also thinks that Byron’s poem is called DON WHAN, because he speaks Spanish and that’s the way the name is pronounced in Spanish. When I told him it’s JEWUN, he told me I was a moron.”
Ron Padgett, Ted: A Personal Memoir of Ted Berrigan (1993)
[“Your Liberal-Arts $ at Work.” Harry Ransom Center, University of Texas at Austin. Click for a larger view.]
Related reading
All David Foster Wallace posts (Pinboard)
E. B. White on W3 (with DFW on Webster’s Third)
How to punctuate a sentence
How to punctuate more sentences