If thinking is about making distinctions, then it can be to our advantage to understand even the finer uses of words and punctuation that sectionize our experience and help us communicate our thoughts accurately. One such is the use of a comma before the conjunction because. Whether or not we include it can change what we intend to say.
Take, for example, this sentence which I happened to read the other day: Sometimes we can’t understand how valuable an unexpected opportunity is because our lives are difficult from day to day. Which of these two very different ideas does that sentence mean: Because our lives are difficult from day to day, sometimes we can’t understand how valuable an unexpected opportunity is, or Sometimes we can’t understand what a valuable and unexpected opportunity it is that our lives are difficult from day to day. Questions like this are answered by beginning what is called a formal analysis, looking closely at the form, that is, the words and punctuation of the statement in question, and on the basis of what we find, conclude then accordingly.
Let’s restate the original sentence to begin afresh: Sometimes we can’t understand how valuable an unexpected opportunity is because our lives are difficult from day to day. From the largest view, taking the statement as a whole, we see that it is a complex sentence: one independent clause (Sometimes we can’t understand), together with two subordinate clauses (how valuable an unexpected opportunity is and because our lives are difficult from day to day). What concerns us here is the second subordinate clause beginning with the conjunction because. As a subordinating conjunction, because regularly has two distinct meanings: since and the fact that. Understanding the logical difference between those two ideas will give us the answer we’re looking for.
Subordinating conjunctions act like logical switches in a sentence; they tell the reader how to connect the thought of one clause logically to the thought of another clause. Some conjunctions, like when, connect one thought to another by way of time: I left when it started to rain; others, like if, stipulate the condition on which something else might happen: I will leave if it starts to rain. In the same way, the conjunction because, just like since, most often represents the idea of cause. Normally, when a subordinate clause follows an independent clause, there is no comma between them, as in the examples just above with when and if. But if the independent clause is negative, as is the case in the sentence we are analyzing, the conjunction because most often will have a comma before it if it intends to indicate cause: Sometimes we can’t understand how valuable an unexpected opportunity is, because our lives are difficult from day to day. This is illustrated more clearly by the first of our revised interpretations.
If, however, the independent clause is negative and there is no comma before because, the logical meaning of the conjunction changes from cause to substantive, meaning that the clause it introduces is intended to stand in its entirety as a noun. Thus Sometimes we can’t understand how valuable an unexpected opportunity is because our lives are difficult from day to day means that what is a valuable opportunity is the fact that our lives are difficult from day to day—probably not a common belief, but in fact just what the original sentence, and our second interpretation differently worded, means.
The rule of thumb, then, is to observe whether or not the independent clause before the subordinating conjunction because is negative. If it is, most often a comma will be necessary.
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