When does the ACT require a comma, and just as importantly, when does it test you by adding a comma that should not be there?
Commas and unnecessary commas on ACT English: the four jobs commas do (separating items in a series, setting off nonessential elements, following introductory elements, and joining clauses with a coordinating conjunction), and recognizing the unnecessary commas the ACT inserts between a subject and verb or around essential information.
A focused answer to commas on ACT English: the four jobs a comma does (series, nonessential elements, introductory elements, joining clauses with a coordinating conjunction) and the unnecessary commas the test plants, such as a comma between a subject and its verb or around essential information, with a routine for deciding.
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What this skill is asking
Commas are the most tested punctuation mark on the ACT, and the section checks both directions: knowing when a comma is required and knowing when one is wrong. The second is what trips students up, because the ACT plants commas that feel natural (a pause where you might breathe) but break a rule, especially the comma that splits a subject from its verb. The skill is to treat commas as having a small number of jobs, and to reject any comma that is not doing one of them.
The four jobs of a comma
Learn the jobs, and you can judge any comma by asking "which job is this doing?"
Essential versus nonessential
The nonessential-element job hides the section's most common comma decision: does this information take commas or not?
The classic example is a name. "My sister, Diana, lives in Denver" implies you have only one sister (her name is extra). "My sister Diana lives in Denver" implies you have more than one, and "Diana" is needed to say which, so it takes no commas. The ACT tests both, and the surrounding context tells you which is meant.
Spotting the unnecessary comma
The single most important habit is refusing the subject-verb comma. A complete subject, however long, is never separated from its verb by one comma.
Why "would I pause here?" is the wrong test
Reading commas by where you would breathe fails on the ACT, because natural pauses do not match the rules: people pause between a long subject and its verb, before a "such as" list, and around essential clauses, all of which take no comma. The reliable method is the job test: a comma must be doing one of the four jobs, and nonessential pairs must have both commas. If you cannot name the job, take the comma out. This habit also supports the run-on topic, where the comma-plus-conjunction job is the only one that joins clauses, and the dashes topic, where a pair of dashes can do the nonessential job instead of commas.
Try this
Q1. What are the four jobs a comma does on the ACT, and what should you do with a comma that is not doing one of them? [Recall]
- Cue. Separate items in a series, set off a nonessential element (comma on both sides), follow an introductory element, and join two independent clauses with a coordinating conjunction. A comma doing none of these jobs is unnecessary and should be removed.
Q2. Explain whether "The author, who won the prize, lives in Maine" is correct if the writer has been discussing one specific author already named. [Short explanation]
- Cue. It is correct. Because the author has already been identified, "who won the prize" is extra, nonessential information, so it takes a comma on both sides. (If the clause were needed to say which author among several, it would be essential and take no commas, and "who" would more naturally be "that" for an essential clause about a thing.)
Exam-style practice questions
Practice questions written in the style of ACT exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
ACT English (style)1 marksChoose the best option: 'The scientist who discovered the vaccine, received a major award.' (A) NO CHANGE (B) vaccine received (C) vaccine; received (D) vaccine: receivedShow worked answer →
The correct answer is (B), "vaccine received". The comma in the original wrongly separates the subject ("The scientist who discovered the vaccine") from its verb ("received"). A subject should never be split from its verb by a single comma.
Why not the others: (A) keeps the subject-verb-splitting comma; (C) a semicolon needs an independent clause on both sides, but "received a major award" is not independent; (D) a colon needs a complete clause before it. The correct version simply removes the comma.
ACT English (style)1 marksChoose the best option: 'My oldest sister Diana lives in Denver, my other sisters live nearby.' Assume the writer has more than one sister. (A) NO CHANGE (B) sister, Diana, lives (C) sister Diana, lives (D) sister, Diana livesShow worked answer →
The correct answer is (A), NO CHANGE. Because the writer has more than one sister, "Diana" is essential information, it tells you which sister, so it takes no commas. The original correctly leaves "sister Diana" without commas.
Why not the others: (B) puts commas around "Diana" as if it were nonessential, but it is essential here; (C) and (D) each add one stray comma, splitting the name or the subject from its verb. Essential identifying information takes no commas.
Related dot points
- Common punctuation traps on ACT English: the deliberate errors the test reuses (a comma splitting a subject and verb, a comma splice, a colon after an incomplete clause, mismatched paired marks, and the strategy of choosing the option with the fewest unjustified marks), and a unifying when-in-doubt-leave-it-out habit.
A focused answer to the recurring punctuation traps on ACT English: the subject-verb comma, the comma splice, the colon after an incomplete clause, mismatched paired marks, and over-punctuation, plus the unifying habit of choosing the option that uses no unjustified mark, with worked diagnosis.
- Run-ons and comma splices on ACT English: recognizing two independent clauses joined with no punctuation (fused) or with only a comma (splice), and applying the four standard fixes (period, semicolon, comma plus a coordinating conjunction, or subordination) to the underlined portion.
A focused answer to run-ons and comma splices on ACT English: how to recognize two independent clauses fused with no punctuation or joined with only a comma, and the four standard fixes (period, semicolon, comma plus a FANBOYS conjunction, or subordination), and how each answer choice maps to one of them.
- Dashes and parentheses on ACT English: using a pair of dashes or parentheses (or a pair of commas) to set off a nonessential element, the matching-pair rule that you cannot open with a dash and close with a comma, and using a single dash to introduce an explanation or list after a complete clause.
A focused answer to dashes and parentheses on ACT English: setting off nonessential information with a matching pair of dashes, parentheses, or commas, the rule that the opening and closing marks must match, and using a single dash like a colon to introduce an explanation or list after a complete clause.
- Colons and semicolons on ACT English: the semicolon joins two independent clauses (or separates complex list items), the colon introduces a list, explanation, or example after a complete clause, and the rule that both require a complete independent clause before them, with the contrast to a comma.
A focused answer to colons and semicolons on ACT English: the semicolon links two independent clauses, the colon introduces a list, explanation, or example after a complete clause, both need a full independent clause before them, and how each differs from a comma, with a routine for choosing the right mark.
Sources & how we know this
- Description of the ACT English Test — ACT, Inc. (2025)
- Preparing for the ACT Test — ACT, Inc. (2025)