A non-restrictive clause adds extra information about a noun that is already identified, and it is set off with a comma on both sides.
In the sentence the river, which flows past the old mill, is wide and slow, the clause can be removed and the sentence still makes sense, so the second comma cannot be left out. Restrictive clauses, by contrast, identify which thing is meant, take no commas, and usually begin with that.
Questions on non-restrictive clauses; that/which typically show a sentence missing its closing comma or carrying an extra one, and ask which edit fixes it. Reading the sentence without the clause is a reliable test.
Sample questions
Pick an answer to see whether it is right — nothing is saved, and nothing needs an account.
Question 1Easier
Which edit should be made to correct the sentence?
The Golden Gate Bridge in San Francisco,which is famous for its rust-colored towers is visited by over ten million tourists annually, according to city records.
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Answer: B — Insert a comma after towers.
The nonrestrictive clause “which is famous for its rust-colored towers” begins with a comma after “Francisco” but never closes, so “Insert a comma after towers.” supplies the missing closing comma before the main verb “is visited.” “Delete the comma after Francisco.” removes the opening comma that sets the clause off, and “Delete the comma after annually.” strips the comma that sets off “according to city records.” “Change which to that.” does not supply the missing closing comma and wrongly makes the clause restrictive.
Question 2Easier
Which edit should be made to correct the sentence?
The cratesthat were damaged in shipping, must be returned to the supplier before the warehouse closes for the day.
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Answer: A — Delete the comma after shipping.
A restrictive clause beginning with “that” should not be separated from the main verb by a comma. “Delete the comma after shipping.” removes the incorrect break between the subject and predicate. “Insert a comma after crates.” incorrectly separates the noun from its restrictive clause, “Change that to which.” leaves mismatched punctuation, and “Insert a comma after supplier.” leaves the original error uncorrected.
Question 3Easier
Which edit should be made to correct the sentence?
The Trans-Siberian Railway, which stretches nearly six thousand miles across Russia connects Moscow to Vladivostok, offering travelers a week-long journey through forests and mountains, according to railway historians.
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Answer: B — Insert a comma after Russia.
The non-restrictive clause beginning with “which” must be enclosed by commas, so “Insert a comma after Russia.” correctly closes the clause. “Delete the comma after Railway.” removes its opening boundary; “Delete the comma after Vladivostok.” incorrectly joins the participial phrase to the main clause; and “Delete the comma after mountains.” incorrectly joins the attribution to the preceding phrase. The comma after Vladivostok sets off the participial phrase, while the comma after mountains separates the trailing attribution.
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