No topic in the craft of English composition raises more questions than the comma. The difficulty lies in the fact that there are so many reasons we may need, or not need, to place a comma somewhere in a sentence: sometimes the comma will help us be clear about what we’re saying, and sometimes it will shape the manner, or style, of our thoughts. These two objectives, logical and rhetorical, are found in the structure of our sentences, and that is why the surest way to understand when and where and why to place a comma is to understand sentence construction.
Let’s look at this simple-seeming sentence: I made a reservation Tuesday, and we left Wednesday. Would you have included a comma? Many of us will turn first to our ear to observe whether we paused after the word Tuesday; if so, we might remember being taught some such rule as put a comma where you breathe, and that, we believe, will answer the question. In fact, though, our ear is too sophisticated a faculty to employ first in answering questions of punctuation; it has its place (as we’ll see in a moment) in matters of style, but where it’s a matter of logic, of sorting out for the reader what we’re talking about, we need to use our eyes, not our ears.
That means, then, that we have to first look closely at, not listen to, the grammatical structure of a sentence. Our example above is one sentence, yes, but it is made up of two independent clauses, which means it has two combinations of a subject and a verb: I made and we left. These two clauses are joined by and, a coordinating conjunction, and with those observations, we have the information we need to decide whether or not to place a comma. For at the very top of the list of comma rules in any grammar book is this: use a comma to separate independent clauses when they are joined by a coordinating conjunction and their subjects are different.
Now we might balk and groan at the mention of independent clauses and coordinating conjunctions, but every art has its elements, difficult sounding at first but really very straightforward once we take the time to look. An independent clause is just that, independent; it needs no other part of the sentence to say what it is saying. I made a reservation Tuesday means what it says; it is complete in and of itself in a way that because I made a reservation Tuesday is not. This because-clause cannot stand on its own; it depends on another idea yet to be mentioned in its sentence, and that is why such dependent clauses are termed subordinate. All clauses are either independent or subordinate. And as for conjunctions, words that join clauses, we have only two large classes as well: coordinating and subordinating. English grammar has boxes of each, but a good handful are enough at first to know: and, but, or, therefore are common coordinating conjunctions, and when, because, if, after, although are subordinating. Begin a clause with a subordinating conjunction, and you’ve just made it a subordinate clause.
Why all this? Because those few elements make the rules work, and knowing the rules gives us, ironically, both independence and confidence. So now, if we happen to revise the original sentence to I made a reservation and I left Wednesday, we have reason to know that a comma is no longer necessary between the two clauses because each clause has the same subject, I. And if we revise the sentence one more time to read I made a reservation, and was finally able to leave early Wednesday afternoon—well, now we’re in new territory, where indeed our ear will be of service. We still have two clauses, but the second has omitted the subject, a stylistic device called ellipsis. And in matters of style, we rightly turn to our ear to discern subtleties our logical eye might miss or even obliterate. In this second revision, that comma after reservation is necessary, even though the subjects are the same, because of the length of the second clause: we need to group its ideas together for the reader, and the comma represents a mental pause that will do just that.
But how do we learn the rules of style? For that there is only one pleasurable way: read and read and read—and listen.