A lot can depend on a simple comma, particularly when the word because is involved. Consider this unassuming sentence: Because he had forgotten to take his key, he had to break a window to get back into his house. Should there be a comma here? Let’s sort this out.
One sure thing about grammar and punctuation is this: you will bring all manner of puzzlement and perplexity down on yourself if you try to find an answer first by asking how something sounds. Your ear is deeply impressionistic; it registers and records and doesn’t ask about the whys and wherefores of what it’s hearing. That is good, excellent even, but it is much too sophisticated (some would even say intuitional) an instrument to answer the basic structural questions that form the platform for a good sentence. That’s the work of science, and science is work for the eyes, not the ears.
So if we step back from the sentence, so to speak, what do we see first? That it is composed of two clauses, one subordinate (because he had forgotten) and one independent (he had to break), and that the subordinate clause comes first. And that observation will be enough to decide in favor of the comma, for the compositional rule is: a comma separates a subordinate from an independent clause when the subordinate clause precedes. (And let’s remind ourselves that because he had forgotten is a subordinate clause because it begins with the subordinating conjunction because.) Sentences of this design are called periodic. Invert the order of the two clauses and you will produce what is called a loose sentence: He had to break a window to get back into his house because he had forgotten to take his key. And what do you notice? The comma has disappeared, which accords with another rule for our eyes: a comma does not separate a subordinate from an independent clause when the independent clause precedes.
That’s that, but what about this: He didn’t break the window because he had forgotten his key. You will say that here again no comma is necessary because the independent clause (he didn’t break) precedes the subordinate (because he had forgotten) and you will be correct—technically. But something else is going on here, because the negative independent clause (didn’t means did not) strongly suggests that another reason exists. Commas cut, which mean they separate, disjoin, disconnect one thing from another, sometimes for clarity and sometimes for logic. And conversely, when there is no comma, the meaning of one clause is deeply implicated in the meaning of the following clause (an idea in grammar called restriction). So here, where there is no comma separating the two clauses (conforming properly to the rule we’ve observed) but the main verb is negative, the cloud of suspicion is so thick you can cut it with your mind: so if forgetting his key wasn’t the reason he broke the window, what was the reason? He didn’t break the window because he had forgotten his key. He broke the window because he was so angry at himself.
The three basic marks of punctuation we rely on are the comma, semicolon, and period, and you would think that with only three workaday marks to worry about (there are many others, of course, for special circumstances), things would be fairly straightforward. But language, strange to say, is in fact a dull knife to cut up and paste together again our living experience. And that’s why these three marks of punctuation are so important to use correctly. The recommendation always is to look first at the structure of the sentence (phrases and clauses), and then ask how one of these elements is related to the next. That will give you a solid stage on which to act—and even dance one day when your ear gets tired of all the rules. One day.
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