The question before us is whether there should be a comma before the word because in this sentence: I didn’t go to the meeting because I was out of town. This seemingly minute problem can remind us that caring about grammar is more than linguistic etiquette. We write to communicate our thoughts, and if the sentences we compose say one thing and we mean another, that’s a problem worth understanding.
Let’s begin by recalling that the ubiquitous word because can work grammatically as either a preposition or a conjunction. As a preposition, it is most often combined with of to form what is called a compound preposition. The two words are regarded as a unit, and a noun follows as the object of the compound unit without a comma: we called the meeting because of a customer’s complaint. More commonly, though, the word because works alone as a conjunction, specifically as a subordinating conjunction. In that role, it introduces (as do all conjunctions) a clause, a group of words with a subject and predicate, and that is how it is functioning in the sentence we are examining: because I was out of town.
Subordinating conjunctions carry out the important (let’s say essential) role of organizing logically the multiple thoughts within a sentence. We are accustomed to thinking about writing and revising in terms of sentences, but our real focus should be on the clauses we compose. A clause, in contrast to a phrase, is a group of words with a subject and predicate. A predicate is initiated with a verb and includes any other words needed to complete the thought, so combining a subject and predicate produces a thought: here’s what I’m talking about (the subject) and here’s what I’m saying about it (the predicate). Now if one sentence has two clauses, it then has two thoughts, and so the question of how the two thoughts are related logically to each other immediately arises.
The many subordinating conjunctions in English are divided into several categories that define the logical connection any particular one is meant to communicate. The most important of these groups, it’s probably safe to say, is reason, or cause, and that is the rubric under which the word because stands. All this just means that when because begins a clause, it intends to tell the reader that the thought which is about to follow is the cause or reason of the thought in the preceding clause. So in the sentence we are looking at, the subordinate clause because I was out of town is meant to explain the reason why I didn’t go to the meeting. That seems obvious, but what is obvious can sometimes make us oblivious.
In our sentence, the first of the two clauses is independent, which means it succeeds in expressing a complete and rounded assertion: I didn’t go to the meeting. The word didn’t, of course, is a contraction for the verb phrase did not, and so we also note that this independent clause is negative: I did not go. When a subordinate clause follows an independent clause, it is usual not to separate the two with a comma, and so at first glance, we would think that all is good and lawful here: the fact that I did not go to the meeting has been attached tightly to its purported cause, I was out of town. But we have also noted that the independent clause is negative—and this is the lair we are stalking. Without a comma, the subordinate clause has become part and parcel of the negative independent clause, and so is negatived itself: I didn’t go to the meeting because I was out of town means that being out of town was not the reason I didn’t go to the meeting. Perhaps I had another appointment, perhaps I just don’t want to tell you, but without a comma, the thought of the subordinate clause is denied along with the thought of the independent clause. So when the independent clause is negative, we should put a comma before because.
Which brings us back to our first observation. The minutiae of grammar can have outsize logical consequences, and we will be hard put to explain why such things are not important when the very meaning we’re trying to communicate can depend on the smallest of black smudges on a line.