Sentence structure questions test whether you can recognize and fix poorly constructed sentences. On the ACT English section, these questions account for roughly 8–12 of the 75 questions you'll face in 45 minutes. They cover a range of issues: sentence fragments, run-on sentences, comma splices, faulty coordination and subordination, parallel structure errors, and misplaced or dangling modifiers.
What makes sentence structure questions challenging is that they require you to understand how the parts of a sentence work together. Unlike punctuation questions, where you can often focus on a single comma or semicolon, sentence structure questions require you to analyze the overall grammatical architecture of a sentence — how clauses connect, whether modifiers are properly placed, and whether the sentence is complete.
The ACT English section uses a passage-based format with underlined portions. For sentence structure questions, the underlined section usually contains the structural error, and the answer choices present different ways to restructure the sentence. Sometimes the question will ask you to choose the most effective combination of sentences or the best way to connect two ideas.
This guide covers every sentence structure concept the ACT tests. You'll learn the rules, see them in action through worked examples, and practice with ACT-style questions. With a solid understanding of these concepts, you'll be able to identify structural errors in seconds — well within the 36-second average you have per question.
Core Concepts
Sentence Fragments
A sentence fragment is a group of words that looks like a sentence but is missing a subject, a verb, or a complete thought. On the ACT, fragments typically appear in two forms:
1. Dependent Clause Standing Alone
A dependent clause has a subject and verb but begins with a subordinating conjunction (because, although, when, if, since, while, after, before, unless, until) and cannot stand alone.
- Fragment: Because the weather was unusually cold.
- Complete: Because the weather was unusually cold, the event was moved indoors.
2. Phrase Without a Main Verb
Participial phrases, infinitive phrases, and prepositional phrases cannot function as complete sentences.
- Fragment: Running through the park at sunrise. (participial phrase — no subject or main verb)
- Complete: She was running through the park at sunrise.
- Fragment: To complete the project before the deadline. (infinitive phrase)
- Complete: The team worked overtime to complete the project before the deadline.
On the ACT, fragments are usually created when a period or semicolon incorrectly separates a dependent clause from the independent clause it belongs to. The fix is to reconnect the fragment to the main clause, often with a comma or by removing the period.
Run-On Sentences (Fused Sentences)
A run-on sentence (also called a fused sentence) occurs when two independent clauses are joined without any punctuation or conjunction.
- Run-on: The train was late we missed our connection.
- Correct: The train was late, so we missed our connection.
- Correct: The train was late; we missed our connection.
- Correct: The train was late. We missed our connection.
Comma Splices
A comma splice occurs when two independent clauses are joined by only a comma — without a coordinating conjunction.
- Comma splice: The museum was crowded, we decided to come back another day.
- Correct: The museum was crowded, so we decided to come back another day. (add FANBOYS)
- Correct: The museum was crowded; we decided to come back another day. (use semicolon)
- Correct: Because the museum was crowded, we decided to come back another day. (make one clause dependent)
The ACT tests comma splices frequently. To spot them, check whether the words on both sides of a comma could each stand alone as complete sentences. If they can, you have a comma splice.
Subordination and Coordination
The ACT tests whether you can properly connect ideas based on their relationship.
Coordination joins ideas of equal importance using coordinating conjunctions (FANBOYS: for, and, nor, but, or, yet, so) or semicolons.
- The experiment succeeded, and the team celebrated. (Two equally important ideas)
Subordination shows that one idea depends on or is less important than another, using subordinating conjunctions (because, although, since, when, if, while, after, before, unless, until).
- Although the experiment was risky, the team decided to proceed. (The risk is secondary to the decision)
Common ACT traps with subordination:
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Double subordination: Using two subordinating conjunctions for the same clause.
- Incorrect: Although the movie was long, but it was entertaining. (Choose "although" OR "but," not both)
- Correct: Although the movie was long, it was entertaining.
- Correct: The movie was long, but it was entertaining.
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Wrong relationship word: Using a conjunction that doesn't match the logical relationship.
- Incorrect: Because she practiced every day, she failed the audition. ("Because" implies a cause-effect that doesn't match)
- Correct: Although she practiced every day, she failed the audition. (contrast relationship)
Parallel Structure
Parallel structure means expressing similar ideas in the same grammatical form. The ACT tests this concept heavily.
In lists:
- Not parallel: The job requires writing reports, analyzing data, and to present findings.
- Parallel: The job requires writing reports, analyzing data, and presenting findings. (all gerunds)
- Also parallel: The job requires the ability to write reports, analyze data, and present findings. (all infinitives)
With correlative conjunctions (not only...but also, either...or, neither...nor, both...and):
- Not parallel: She is not only a talented musician but also excels at painting.
- Parallel: She is not only a talented musician but also a skilled painter. (noun phrase + noun phrase)
In comparisons:
- Not parallel: Swimming in the ocean is more challenging than a pool.
- Parallel: Swimming in the ocean is more challenging than swimming in a pool. (gerund phrase compared to gerund phrase)
Rule of thumb: Whatever grammatical form follows the first part of a parallel structure must match the form after every subsequent part.
Misplaced Modifiers
A misplaced modifier is a word or phrase that is too far from the word it modifies, creating confusion or unintended meaning.
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Misplaced: She nearly drove the car for six hours. (Did she nearly drive, or did she drive for nearly six hours?)
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Correct: She drove the car for nearly six hours.
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Misplaced: The professor only teaches on Tuesdays. (Does the professor only teach, or only on Tuesdays?)
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Correct: The professor teaches only on Tuesdays.
Adverbs like "only," "nearly," "almost," "just," "even," and "merely" should be placed directly before the word they modify.
Dangling Modifiers
A dangling modifier is an introductory phrase that doesn't logically modify the subject of the main clause. This is one of the ACT's favorite sentence structure traps.
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Dangling: Walking through the museum, the paintings impressed the visitors. (This says the paintings were walking through the museum.)
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Correct: Walking through the museum, the visitors were impressed by the paintings. (Now "the visitors" are the ones walking.)
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Dangling: After studying for weeks, the test seemed easy. (The test didn't study for weeks.)
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Correct: After studying for weeks, she found the test easy.
ACT Rule: The subject immediately following an introductory modifying phrase must be the person or thing performing that action. If it's not, the modifier is dangling.
Strategy Tips
Tip 1: Check Both Sides of Every Comma
When you see a comma in the underlined portion, ask: "Could the words on both sides stand alone as complete sentences?" If yes, you have a comma splice. You need either a semicolon, a period, or a comma with a FANBOYS conjunction.
Tip 2: Read the Introductory Phrase, Then Check the Subject
For modifier questions, read the introductory phrase (everything before the first comma), then look at the subject immediately after the comma. Ask: "Is this subject logically doing the action in the introductory phrase?" If not, you have a dangling modifier.
Tip 3: Scan Lists for Parallel Form
Whenever you see a list of two or more items, check that all items share the same grammatical form. If the first item is a gerund ("-ing" word), all items should be gerunds. If the first is a noun, all should be nouns.
Tip 4: Watch for Double Conjunctions
The ACT loves pairing "although" with "but" or "because" with "so" in the same sentence. Only one conjunction is needed — using both creates a redundant structure that's always wrong.
Tip 5: Try Each Choice as a Standalone Sentence
If a question gives you options that restructure a sentence, mentally read each choice as a complete sentence. Eliminate any choice that creates a fragment, run-on, or comma splice.
Tip 6: Use the "Because" Test for Fragments
If a group of words starts with a subordinating conjunction and ends with a period, it's a fragment. "Because the store was closed" is not a sentence. It needs an independent clause to lean on.
Worked Example: Example 1
The city council voted to increase funding for public parks. _____ the parks had fallen into disrepair over the past decade.
(A) NO CHANGE (B) parks, because the (C) parks because the (D) parks; the
The second sentence "Because the parks had fallen into disrepair over the past decade" is a dependent clause — it's a fragment. It cannot stand alone as a sentence after a period. We need to connect it to the independent clause. Choice (C) joins them correctly as one sentence without a comma before "because" (which is standard when "because" follows the main clause). Choice (B) adds an unnecessary comma. Choice (D) creates a structure that loses the causal relationship. Answer: (C)
Worked Example: Example 2
The new software streamlined operations, it reduced processing time by nearly 40 percent.
(F) NO CHANGE (G) operations; it reduced (H) operations it reduced (J) operations, it reducing
The original is a comma splice — two independent clauses joined by only a comma. Choice (G) fixes this with a semicolon between the two independent clauses. Choice (H) creates a run-on (fused sentence). Choice (J) creates a fragment in the second part ("it reducing" is not a complete verb form). Answer: (G)
Worked Example: Example 3
Hoping to improve her marathon time, a new training regimen was adopted by Sarah that included interval sprints and hill workouts.
(A) NO CHANGE (B) Sarah adopted a new training regimen that included interval sprints and hill workouts. (C) a new training regimen, including interval sprints and hill workouts, was adopted by Sarah. (D) adopting a new training regimen was done by Sarah, including interval sprints and hill workouts.
The introductory phrase "Hoping to improve her marathon time" is a dangling modifier. The subject after the comma must be the person doing the hoping — Sarah. In choice (A), the subject is "a new training regimen," which can't hope. Choice (B) places "Sarah" as the subject immediately after the introductory phrase: "Hoping to improve her marathon time, Sarah adopted..." This is correct. Choice (C) still has "regimen" as the subject. Choice (D) is awkwardly passive. Answer: (B)
Worked Example: Example 4
The internship program requires participants to attend weekly seminars, complete a research project, and _____ a final presentation to the department.
(F) they must deliver (G) delivering (H) deliver (J) the delivery of
This is a parallel structure question. The list contains three items after "to": "attend," "complete," and the third item must match. Both "attend" and "complete" are base-form verbs (infinitives without the repeated "to"). The parallel form is "deliver" — another base verb. Choice (F) breaks parallelism by adding "they must." Choice (G) uses a gerund. Choice (J) uses a noun phrase. Answer: (H)
Worked Example: Example 5
Although the evidence strongly supported the defendant's innocence, _____ the jury still deliberated for three days.
(A) NO CHANGE (B) but the jury still deliberated for three days. (C) and the jury still deliberated for three days. (D) however, the jury still deliberated for three days.
"Although" is a subordinating conjunction that already establishes a contrast relationship. Adding "but" (B) creates double subordination — using two contrast words for the same relationship. Adding "and" (C) doesn't convey contrast. Adding "however" (D) is redundant with "although." Choice (A) correctly uses only "although" to introduce the dependent clause and follows with the independent clause. Answer: (A)
Practice Problems
Problem 1
The researchers published their findings, the results confirmed what many scientists had long suspected about climate patterns.
(A) NO CHANGE (B) findings; the results (C) findings the results (D) findings, the results which
Problem 2
Perched on the highest branch of the old oak tree, _____ could barely be seen from the ground below.
(F) the eagle's nest (G) the nest belonging to the eagle (H) the eagle had built a nest that (J) an eagle's nest, which
Problem 3
The scholarship program aims to identify promising students, provide them with financial support, and _____ them throughout their college careers.
(A) is mentoring (B) mentor (C) the mentoring of (D) to be mentoring
Problem 4
Because the bridge had been damaged by the storm. _____ was rerouted through the neighboring town.
(F) Traffic, in the meantime, (G) Traffic (H) Therefore, traffic (J) So traffic
Problem 5
The film not only captivated audiences with its stunning visuals _____ challenged viewers to reconsider their assumptions about artificial intelligence.
(A) and (B) but also (C) but it (D) while it also
Answers: 1. (B) — The original is a comma splice; a semicolon correctly separates two independent clauses. 2. (F) — The introductory phrase "Perched on the highest branch" modifies something sitting there — "the eagle's nest." Choice (H) restructures so the eagle is the subject but creates a fragment. 3. (B) — Parallel structure with "identify" and "provide" requires the base verb "mentor." 4. (G) — The first clause "Because the bridge had been damaged by the storm" is a dependent clause, so joining it to an independent clause "Traffic was rerouted" fixes the fragment. "Therefore" (H) and "So" (J) would be redundant with "Because." 5. (B) — "Not only...but also" is the correct correlative conjunction pair. Choice (A) breaks the pattern, and (C) and (D) don't complete the correlative pair.
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Common Mistakes
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Ignoring comma splices. Students often think a comma is sufficient to join two sentences, especially when the second sentence is short or closely related to the first. On the ACT, a comma splice is always wrong — you need a semicolon, a period, or a comma plus coordinating conjunction.
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Not checking the subject after an introductory phrase. Dangling modifier questions are among the most commonly missed on the ACT. Always verify that the subject immediately after the comma logically performs the action described in the introductory phrase.
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Breaking parallel structure without realizing it. When lists contain many words, it's easy to lose track of the grammatical pattern. Slow down on list questions and verify that every item follows the same form — all gerunds, all infinitives, or all nouns.
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Using double conjunctions. "Although...but," "because...so," and "since...therefore" are always wrong. Choose one conjunction per relationship. The ACT tests this pattern repeatedly.
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Creating fragments by over-separating ideas. When a dependent clause is separated from its independent clause by a period or semicolon, it becomes a fragment. Dependent clauses (starting with because, although, when, if, since, while, etc.) must be attached to an independent clause.
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Misplacing "only" and similar adverbs. "The teacher only assigns homework on Fridays" means something different from "The teacher assigns homework only on Fridays." Place these adverbs directly before the word they modify.
Frequently Asked Questions
How many sentence structure questions are on the ACT?
Expect 8–12 questions testing sentence structure out of the 75 total. These include fragments, run-ons, comma splices, parallel structure, coordination/subordination, and modifier placement. Combined with punctuation and other grammar questions, Conventions of Standard English makes up roughly 50–60% of the ACT English section.
What's the difference between a run-on sentence and a comma splice?
A run-on (fused sentence) has no punctuation between two independent clauses: "The rain stopped the sun came out." A comma splice has only a comma: "The rain stopped, the sun came out." Both are incorrect, but they require different fixes. A run-on needs punctuation added; a comma splice needs the comma replaced with a stronger punctuation mark or supplemented with a conjunction.
How do I fix a dangling modifier on the ACT?
Make sure the subject of the main clause is the person or thing performing the action in the introductory phrase. If "Exhausted from the hike" opens the sentence, the subject must be a person (not "the trail" or "the campsite"). On the ACT, look for the answer choice that places the correct logical subject right after the introductory comma.
Is it ever okay to start a sentence with "because" on the ACT?
Yes — as long as the "because" clause is properly connected to an independent clause. "Because the store was closed, we went home" is a perfectly correct sentence. What's wrong is ending the sentence after the dependent clause alone: "Because the store was closed." (fragment).
How can I quickly identify parallel structure errors?
Look for lists, pairs, or comparisons. Identify the grammatical form of the first item (noun, gerund, infinitive, adjective, etc.), then check that every subsequent item matches. On the ACT, the correct answer will always maintain consistent grammatical form across all items.
Key Takeaways
Every sentence needs a subject, a verb, and a complete thought. If any of these is missing, you have a fragment. Watch for dependent clauses and participial phrases masquerading as sentences.
Two independent clauses need proper joining. You can use a period, a semicolon, or a comma + FANBOYS. A comma alone (comma splice) or nothing at all (run-on) is always wrong.
One conjunction per relationship. Don't pair "although" with "but" or "because" with "so." The ACT tests this trap frequently — choose one subordinating conjunction or one coordinating conjunction, never both.
Parallel structure means matching forms. In lists, comparisons, and correlative conjunction pairs, every element must share the same grammatical structure. Check gerunds with gerunds, infinitives with infinitives, nouns with nouns.
The subject after the comma must match the introductory modifier. For dangling modifier questions, identify who or what is performing the action in the opening phrase, then make sure that person or thing is the subject of the main clause.
"Only," "nearly," and "almost" must sit next to what they modify. Misplaced adverbs change meaning and are a frequent ACT trap. Place these words directly before the element they modify.
When in doubt, simplify. The ACT rewards clear, concise sentence structure. Among grammatically correct options, choose the one that expresses the idea most directly and efficiently.