How do you spot a run-on or comma splice on the ACT, and which of the four standard fixes does each answer choice represent?
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.
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What this skill is asking
If a fragment is too little sentence, a run-on is too much: two complete sentences shoved together. The ACT tests this constantly, in two flavors, the fused run-on (no punctuation between the clauses) and the comma splice (only a comma between them). The good news is that both have the same four fixes, and each answer choice usually represents one of them, so once you recognize "two independent clauses", choosing the right option is mechanical.
Recognizing a run-on
The test for a run-on is simple: are there two independent clauses, and are they joined correctly? If you can split a string into two parts that could each be a sentence, and they are joined by nothing or by only a comma, it is a run-on.
A useful check: cover everything before the suspected join and ask "is this a complete sentence?", then cover everything after and ask the same. If both halves are complete sentences, you have two independent clauses, and the punctuation between them must be one of the correct joins.
The four fixes, and what each looks like in the options
The four fixes give the ACT four natural ways to write a correct option, so the choices often map to them.
Note one trap: words like however, therefore, moreover, thus (conjunctive adverbs) are not coordinating conjunctions. "I trained hard, however I lost" is still a comma splice. To use "however" between clauses you need a semicolon: "I trained hard; however, I lost."
Choosing the fix on an underlined portion
When the underlined portion sits at the join between two clauses, work the four-fix checklist.
Why the four-fix frame is so reliable
The reason run-on questions feel easy once you practice is that they are closed: there are only four correct ways to handle two independent clauses, and the ACT writes wrong options by leaving the clauses fused, splicing them with a comma, or using a conjunctive adverb with only a comma. So your job is not to be creative, it is to recognize "two independent clauses", then scan the options for the one that applies a real fix. This frame also connects to the punctuation module: the semicolon fix is exactly the semicolon rule, and the comma-plus-conjunction fix is exactly the comma rule for joining clauses.
Try this
Q1. What are the four standard ways to fix a run-on or comma splice between clauses A and B? [Recall]
- Cue. (1) A period: "A. B." (2) A semicolon: "A; B." (3) A comma plus a coordinating conjunction: "A, and/but/so B." (4) Subordination: make one clause dependent, "Because A, B." Any of these correctly handles two independent clauses.
Q2. Explain why "The film was long, however, it kept my attention" is still incorrect, and fix it. [Short explanation]
- Cue. "However" is a conjunctive adverb, not a coordinating conjunction, so a comma is too weak to join the two independent clauses around it; this is a comma splice. Fix it with a semicolon: "The film was long; however, it kept my attention." (Or split into two sentences.)
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 lab ran late, the experiment needed another hour.' (A) NO CHANGE (B) late the experiment (C) late; the experiment (D) late, so the experiment, neededShow worked answer →
The correct answer is (C), "late; the experiment". The original is a comma splice: "The lab ran late" and "the experiment needed another hour" are both independent clauses, and a comma alone cannot join them. A semicolon correctly links two related independent clauses.
Why not the others: (A) keeps the splice; (B) removes the comma entirely, creating a fused run-on; (D) adds "so" (a valid fix) but then inserts a wrong comma after "experiment", separating the subject from its verb. Only (C) joins the clauses correctly.
ACT English (style)1 marksChoose the best option: 'Maria trained for months she finished the marathon easily.' (A) NO CHANGE (B) months, she finished (C) months, so she finished (D) months, she, finishedShow worked answer →
The correct answer is (C), "months, so she finished". The original is a fused run-on: two independent clauses with no punctuation between them. Adding a comma plus the coordinating conjunction "so" joins them correctly and shows the cause-and-effect relationship.
Why not the others: (A) is the fused run-on; (B) adds only a comma, creating a comma splice; (D) adds stray commas around "she finished" without fixing the join. A comma plus a coordinating conjunction (FANBOYS) is the correct fix here.
Related dot points
- Complete sentences and fragments on ACT English: a clause needs a subject and a finite verb and must express a complete thought, recognizing fragments created by missing verbs, -ing verbs without a helper, and stray subordinators, and fixing an underlined portion to form a complete sentence.
A focused answer to complete sentences and fragments on ACT English: a sentence needs a subject and a finite verb and a complete thought, how fragments arise from missing or -ing verbs and stray subordinators, and how to fix an underlined portion that leaves a sentence incomplete. The foundation of the sentence-structure questions.
- Joining clauses on ACT English: the three connector types (coordinating conjunctions with a comma, subordinating conjunctions that make a clause dependent, and conjunctive adverbs that need a semicolon), how each is punctuated, and choosing the connector whose logical relationship and punctuation are both correct.
A focused answer to joining clauses on ACT English: coordinating conjunctions (comma before, FANBOYS), subordinating conjunctions (make one clause dependent), and conjunctive adverbs (semicolon before, comma after), how each is punctuated, and how to choose the connector whose logic and punctuation are both right.
- 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.
- 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.
Sources & how we know this
- Description of the ACT English Test — ACT, Inc. (2025)
- Preparing for the ACT Test — ACT, Inc. (2025)