There is only one way to understand the rules about something, and that is to understand the reasons behind them. Let’s take up the comma once again, this time seeing how it separates, or doesn’t separate, two assertions about the same subject.
Let’s begin with this sentence: I found a flight, and was able to leave early Wednesday afternoon. Should there be a comma in that sentence or not? To find an answer, we go first to the fundamental question that lies behind any sentence we read or write: what is the sentence about? Our answer will produce the subject, and knowing the subject will give us a way to design its other elements. A subject, remember, is not just any noun we can find in a sentence. We can’t say that the subject of this sentence, for example, is flight or Wednesday afternoon. A subject must be a noun or pronoun about which something is being said. Nothing is being said about those two nouns, but something is being said—two things, in fact—about the pronoun I: I found a flight and I was able to leave early Wednesday afternoon.
When we combine a subject with what is being said about it, we produce a clause. The sentence we’re examining, then, has two clauses, but the subject, I, has been expressed only once. That’s important to note, because omitting a word that is otherwise grammatically necessary, a phenomenon called ellipsis, plays a role in determining how we punctuate. Had the sentence been written I found a flight and I was able to leave early Wednesday afternoon, there would be no question of a comma to ponder, because a sentence that joins with the conjunction and two clauses about the same subject does not separate those clauses with a comma. But what do we do when one of the clauses omits the subject?
Here’s where a little grammatical terminology will help us sort things out. We said that a clause combines a subject with what is being said about the subject. We say things with verbs, and that part of a clause that includes a verb to say something about the subject is called the predicate. So a clause, then, is the combination of a subject and a predicate. A simple predicate makes only one assertion about a subject; a compound predicate makes more than one assertion. That said, if we return now to our original sentence, I found a flight, and was able to leave early Wednesday afternoon, should we see only one clause with a compound predicate, or two clauses, the second of which has omitted the subject?
A distinction without a difference, you might object, but not as far as punctuation is concerned. The comma after flight is necessary here, not because of the omission of the subject I before was able, but precisely because of the length of that second clause. A good argument could be made that no comma would be necessary had the sentence read, I found a flight and left Wednesday afternoon. That’s just one simple sentence with a compound predicate. But to refer to the fact that the subject was able builds the complexity of the idea, as does too the addition of the adverb early. These further ideas extend the thought, and the reader will better understand that elaborated idea and its implications if the second clause is isolated by a comma.
Our conclusion, then, is that an elliptical clause should be set off by a comma. And if you find an exception to that observation in the hands of a good writer, there will be a reason for it if you look closely enough—probably to be found in the rhetoric, the style and intention, behind the words you’re reading. The reasons behind the rules—and their exceptions—is what matters.
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