The comma pair that changes what a sentence means
"The chemists, who wore goggles, finished the trial" says every chemist wore goggles. Remove the commas and it says only the goggled ones finished. Same words, different claim.
Why it happens
The commas look decorative, so students add or drop them by ear. They are not decorative — they mark information as removable. If a phrase narrows down which thing you mean, it cannot be removable, and it cannot take commas.
See the trap
The paragraph has already said that Okonkwo published four novels between 1991 and 2004. "The novel ___ she published in 1997 ___ won the national prize."
✗ The trap: "The novel, she published in 1997, won" — commas mark the phrase as removable, but she wrote four novels, so it is the only thing identifying which one.
✓ The fix: "The novel she published in 1997 won the national prize." No commas. Had the paragraph said she published exactly one novel, the date would be a bonus fact and both commas would go in.
How to actually fix it
- Delete the phrase and reread the sentence.
- If the sentence now points at something vaguer or different, the phrase is essential — no commas.
- If the sentence still identifies the same thing, the phrase is extra — commas on both sides.
Made this mistake on a real question?
Practice Standard English Conventions questions here. Miss one and it is logged with the exact error, then brought back until you beat it.
Practice Standard English Conventions →FAQ
Is that/which a reliable shortcut?
It is a real convention — "which" takes commas, "that" does not — but the SAT usually gives you the same word in every choice and varies only the punctuation, so you still have to reason from meaning.
How do I stop repeating this mistake?
Reading the explanation isn't enough — your brain needs to beat the trap on a fresh question. Practice the skill in the kaoshen.co question bank: miss one and it is logged with the exact error, then brought back to you three days later and again after fourteen.