I ordered a used book online the other day, and the description of the item began with this sentence: A copy that has been read, but remains in clean condition. The statement is a fragment, of course, and that’s just fine, given the restricted, commercial context in which it appears. But what about the comma? And just what is the conjunction but showing a contrast between?
It is good practice to first restore a fragment before analyzing it, and this sentence has two elements missing. If we take the first half, a copy that has been read, we are told something about the copy, but we are not told what the copy is a copy of. So to analyze, we need to say the obvious: This book is a copy that has been read. Likewise, in the second half of the sentence, but remains in clean condition, we can easily supply the missing subject, but in analysis, we need to actually see it, if only in the form of a pronoun: but it remains in clean condition. So the complete sentence, plausibly faithful to the original, which we now have under our grammatical microscope is: This book is a copy that has been read, but it remains in clean condition.
Now this reconstructed sentence is an acceptable grammatical specimen: two clauses joined by but, a coordinating conjunction that is meant to direct our attention to a contrast of some kind. A coordinating conjunction must refer to like grammatical elements (to coordinate means to bring together things of the same order), and so the independent clause it remains in clean condition must be in contrast to all that precedes it: this book is a copy that has been read. The coordinating conjunction but, preceded by a comma, contrasts the two halves of the sentence. That is what we understand, and that is certainly what the writer meant to say.
A problem arises, though, when the writer simply truncates this complete version, retaining the conjunction but and its preceding comma. In the original fragment, a copy that has been read, but remains in clean condition, the conjunction but does not function properly because there are not two independent clauses for it to contrast; the clause that has been read is a subordinate clause because it begins with the relative pronoun that. As it stands, then, the conjunction but is pointing to something that is not there, namely, the independent clause that we had to restore for our analysis, this book is. And that is the reason this fragmented version falters.
So what to do? If we simply remove the comma from the original statement, balance is restored, for now the coordinating conjunction but will contrast two predicates (two like elements) of the one subordinate clause: A copy that has been read but remains in clean condition would be a properly truncated version of This book is a copy that has been read but remains in clean condition. When a predicate has two or more verbs, it is called a compound predicate, and this is an efficient way to say a number of things about the same subject. But English grammar requires that the verbs in a compound predicate not be separated by a comma, and this was the problem with the original sentence. By deleting the comma, we correct the alignment of ideas and balance the assertion according to its intended meaning.
We may know, or think we know, what a writer logically intends to say, the grammatical construction notwithstanding. But one of the standards of clear writing is that grammar not obscure meaning—no matter how tempted we might be simply to dismiss the intricacies by saying, “Well, you know what he meant.” Sometimes, perhaps, and sometimes not.