ACT English: That vs. Which—Restrictive vs. Nonrestrictive Clauses

"That" introduces a restrictive clause (essential information—clause cannot be removed without changing meaning). "Which" introduces a nonrestrictive clause (extra information—clause can be removed without changing meaning). Example: "The car that is blue belongs to me" (essential—which car? the blue one). "My car, which is blue, belongs to me" (extra—I'm just telling you it's blue). Quick test: Can you remove the clause without losing meaning? If yes, use "which" with commas. If no, use "that" without commas.

Note: British English sometimes uses "which" for both. ACT follows American English rules: "that" for restrictive, "which" for nonrestrictive.

Commas around "which" clauses are mandatory because the clause is extra. Example: "The report, which was long, contained important data." Remove "which" clause: "The report contained important data." Still makes sense; the clause is nonrestrictive. No commas with "that" because it is essential. Example: "The report that was long contained important data." Remove "that" clause: "The report contained important data." Meaning changed; we lost which report. If you see a "which" without commas in an ACT sentence, it is likely an error. Similarly, "that" with commas is usually wrong.

Pattern: which=nonrestrictive=commas. that=restrictive=no commas.

Sentence 1 (Error): "The students which studied hard passed the test." Correction: "The students that studied hard passed the test." (Essential—which students? those that studied hard.) Sentence 2 (Correct): "My sister, which is a doctor, works downtown." Error! Should be: "My sister, who is a doctor, works downtown." (Nonrestrictive, but use "who" for people.) Sentence 3 (Correct): "The painting that hangs in the museum is famous." Essential—which painting? the one that hangs in the museum. Correct. During practice, mark every "that" and "which" and verify punctuation matches meaning.

Build this habit: see "which" = check for commas. See "that" = check no commas.

This error appears in 1-2 of the 75 ACT English questions. Many students confuse the two because they sound similar and both introduce clauses. However, the rule is clear: restrictive=that, nonrestrictive with commas=which. Mastering this distinction is a 10-minute study effort that yields 1-2 guaranteed points.

Learn this rule once and verify it on every practice test until it becomes automatic.