SAT — sentence boundaries and clause punctuation

Standard English Conventions questions on the real test print four versions of one sentence and ask which is correct. These ten work the other way round: they print the sentence once and ask you to name the fault and the repair in words. That is partly a constraint of this format — four sentences differing only by a comma are indistinguishable to an automatic checker — and partly an advantage, because a candidate who can say why a semicolon is wrong there can apply the rule to a sentence nobody has shown them. The rules covered are the ones that decide most marks in this domain: the boundary between one sentence and the next, and what may stand at that boundary.

Nine sentences are faulty and one is not, and the faults are all different. Two clauses joined by a comma alone make a comma splice; joined by nothing at all, a run-on; joined by "however" with commas on both sides, the same error wearing a respectable coat, since a conjunctive adverb is not a conjunction and cannot do a conjunction's work. A subordinating word at the front of a clause holds the whole thing in suspense, and what looks like a long sentence turns out to be waiting for a main clause that never arrives. A semicolon requires a complete sentence on each side; a colon requires one only on the left, which is why "the three materials the builders relied on were:" fails and needs no punctuation at all in that position.

Several of the wrong answers are the instructive part. A run-on tempts a reader toward placing a comma where the two sentences meet, which identifies the right spot and produces a comma splice — one error traded for another. A fragment tempts a reader toward adding the comma that a long introductory clause would need, which punctuates the fragment more neatly without making it a sentence. And a supplement opened with a comma and closed with a dash tempts a reader toward the observation that the comma sits between a subject and its verb, which is true, and which happens only because a supplement was opened there; deleting that comma alone leaves the dash stranded.

The last question is the one worth doing twice. Its stem states what the writer means — that only some of the passengers had bought in advance — and the sentence puts commas around the clause that says so. Those commas mark the clause as incidental, which makes the statement apply to everyone, and the sentence now reports something the writer did not intend. It is the rare punctuation error that changes the meaning rather than the manner, and recognizing it is worth more than any rule learned by heart.

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• Test whether a string of words is an independent clause before deciding what may join it
• Tell a comma splice from a run-on, and name a repair that fixes either
• Recognize a subordinating word as what turns a clause into a fragment
• Apply the rule that a semicolon needs a complete sentence on both sides and a colon only on the left
• Refuse a comma between a subject and its verb, and see when one is there for another reason
• Punctuate a conjunctive adverb such as however joining two independent clauses
• Close a supplementary element with the same mark that opened it
• Decide whether a clause identifies or merely adds, and punctuate it accordingly
• See when a pair of commas changes what a sentence says rather than how it reads
• Reject a repair that removes one error by introducing another

The writer means that only some of the passengers had bought tickets in advance. "The passengers, who had bought tickets in advance, were allowed to board first." Which choice best describes the error in the writer's sentence, and how should it be repaired?