Let’s return to the practical matter of identifying and revising a sentence that is not structurally sound. Something is wrong with the following sentence, for example, even though we can certainly wrest a meaning from it: Interest rates finally came down I decided to look for a new house. When we’re writing fast, in a draft or in a document we don’t judge that important, we’re inclined to let down our grammatical guard. What results here is called a run-on sentence, appropriately named because our thoughts just run on without any signals that would help the reader comprehend our thoughts with less effort. How much the reader has to work to understand what we’ve written affects how persuasive we will be.
The examination of almost every sentence we put under the linguistic microscope should begin with finding the join between subject and predicate. Those two sections make up a clause, and thereby also make up every thought we assert. If we apply that technique to the present example, the problem the sentence has quickly comes to evidence. Interest rates are what the sentence begins to talk about, and so those two words make up the subject of—not the sentence but, as we’ll see, the first clause. With that subject works the verb came down, and if we put that subject and predicate together, we have an independent clause: Interest rates finally came down.
Now no sooner did the writer write down that independent thought than he had something further to say. The pronoun I establishes a second subject, and its verb, decided, follows immediately. Those two words, along with the six-word adverbial phrase which modifies that verb, all comprise a second independent clause within the bounds of one compound sentence. And that is just the fact we’re looking for, because the structural laws of English grammar require some marker to connect two independent clauses laid next to each other like that. Absent such connector, a run-on sentence results.
Now the first temptation which usually presents itself in this situation is simply to add a comma: Interest rates finally came down, I decided to look for a new house. That, however, is not an option when a sentence comprises only a pair of independent clauses, and that misguided choice ends up producing another error called a comma-splice, another appropriately named violation because the comma does indeed splice—and not logically connect—the two thoughts set down cheek by jowl (though we’ll see an exception to this in a moment).
So what to do? We have at least three acceptable revisions. The first is to join the two independent clauses with a coordinating conjunction, the most common of which is and: Interest rates finally came down, and I decided to look for a new house. The connector and is the neutral, most noncommittal of conjunctions, leaving the reader to impute a logical relationship between the thoughts. Notice, importantly, that the comma remains with this conjunction when the subjects of the two clauses, as here, are different. (Were the subjects the same, no comma would be in order: I saw that interest rates came down and I decided to look for a new house.) Or if we did not wish to leave it to the reader to determine the logical connection between the thoughts, we could use another coordinating conjunction to be more specific: Interest rates finally came down, so I decided to look for a new house, where so signals what resulted from the preceding thought.
Our second revision is the converse of the first. Instead of adding a conjunction, we omit it, but replace the omitted conjunction with a semicolon (not a comma): Interest rates finally came down; I decided to look for a new house. That brings the reader back into the work of thinking out the logic between the two clauses, but does so under the aegis of the semicolon, which controls the separation of the thoughts, suggesting the logic to the reader and preventing a slippery run-on construction.
And, finally, the simplest and often the most overlooked option is just to sharply separate the clauses of one compound sentence into two simple sentences, each an independent thought in its own right: Interest rates finally came down. I decided to look for a new house. Not often the most graceful of solutions, but still at times effective when a decisive tone is needed.
Much more could be said about each of these solutions, because which would be best depends upon the audience and purpose of what we are writing. And, of course, there are exceptions, even to one of the problems presented here. When the two clauses of a compound sentence are short, sometimes a comma is sufficient: He growled, I ran. But what’s a rule without an exception?
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