ACT English Sentence Structure: Fragments and Modifiers

By Muntasir Jul 28, 2026 8 min read

TL;DR

Sentence Structure and Formation questions sit inside Conventions of Standard English, which accounts for 38 to 43 percent of the English section's questions. The sub-skill tests four error types: fragments and run-ons, comma splices, modifier placement, and parallel structure. A fragment is missing a subject, a verb, or a complete thought, and a run-on jams two full sentences together with no punctuation. A comma splice joins two full sentences with only a comma, which is not enough. A misplaced modifier describes the wrong word because it sits too far from the word it should describe. Parallel structure means matching grammatical form across items in a list or comparison.

ACT English Sentence Structure: Fragments and Modifiers

Sentence structure questions ask you to spot a broken sentence and pick the revision fixing it. ACT groups this work under Sentence Structure and Formation, one of three sub-skills inside the Conventions of Standard English reporting category. The official description states the skill as the ability to "apply understanding of sentence structure and formation in a text and make revisions to improve the writing." Conventions of Standard English accounts for 38 to 43 percent of the English section and splits into Sentence Structure and Formation, Punctuation, and Usage.

This guide covers the four error types tested under Sentence Structure and Formation: fragments and run-ons, comma splices, modifier placement, and parallel structure. Each section states the rule, then shows a broken sentence next to a fixed one. The examples are original, built to match the sentence patterns the test uses.

How the English Test Presents These Questions

The English test gives you passages with underlined or highlighted portions. Most questions ask you to pick the best revision for the underlined part. A smaller share asks for the choice marked NOT or LEAST acceptable, which reverses your usual approach. "No Change" is always one of the answer choices, so leaving a correct sentence alone is a valid answer.

ACT does not publish a further split of how many Sentence Structure and Formation questions target fragments versus modifiers versus parallel structure. Treat every sentence in a passage as a candidate for one of these four errors, since the test does not label which rule a question is testing. The section runs 50 questions in 35 minutes, with 40 of them scored, so your check on each underlined portion has to be fast.

Fragments and Run-ons

A complete sentence needs a subject, a verb, and a complete thought. A fragment is missing one of these three pieces and cannot stand alone. A run-on has the opposite problem: two complete sentences packed into one unit with nothing joining them. Both errors show up in the same underlined portion, so the first move is the same for each.

Spotting a Fragment

Fragments often start with a subordinating word like "because," "although," or "since," or they drop the verb entirely. Read the underlined portion by itself, separate from the rest of the passage, and check whether it forms a full statement. If the words leave you waiting for the rest of the idea, you have a fragment.

Broken: "Because the museum extended its hours." This clause depends on another idea and cannot stand alone. Fixed: "The museum extended its hours, so attendance grew." Adding a main clause completes the thought.

Spotting a Run-on

A run-on jams two complete sentences together with no punctuation or connecting word between them. Unlike a fragment, a run-on has too much packed into one unit rather than too little. Test for it by splitting the underlined portion at the point where the idea shifts.

Broken: "The library closed early the staff needed time to reshelve books." Two full sentences sit back to back with nothing joining them. Fixed: "The library closed early because the staff needed time to reshelve books." A subordinating word turns the second clause into a dependent one. The table below lists the fix options for both fragments and run-ons.

Error	Fix	Example
Fragment	Attach it to an independent clause	"Because the museum extended its hours, attendance grew."
Fragment	Add the missing subject or verb	"The committee reviewed the plan" instead of "The committee, reviewing the plan."
Run-on	Split into two sentences with a period	"The library closed early. Staff needed time to reshelve books."
Run-on	Add a subordinating or coordinating word	"The library closed early because staff needed time to reshelve books."

Comma Splice Fixes

A comma splice is a specific type of run-on. It joins two complete sentences with only a comma, and a comma alone cannot connect two full sentences. Broken: "The bridge reopened last week, traffic returned to normal within days." Both halves are complete sentences, so a comma is not strong enough to join them. Four fixes repair a comma splice, and each one works for a different sentence.

Period. Split the two ideas into separate sentences: "The bridge reopened last week. Traffic returned to normal within days."
Comma plus coordinating word. Add "and," "but," "so," or another coordinating conjunction after the comma: "The bridge reopened last week, and traffic returned to normal within days."
Subordinating word. Turn one clause into a dependent clause: "After the bridge reopened last week, traffic returned to normal within days."
Semicolon. Join the two full sentences with a semicolon and no coordinating conjunction.

Reading each half of the sentence on its own tells you whether a comma splice is present. If both halves form a full sentence, a plain comma between them is wrong. If one half is a dependent clause, the comma is correct and "No Change" is the likely answer.

Dangling and Misplaced Modifiers

A modifier describes another word in the sentence, and it needs to sit close to the word it describes. Placing it in the wrong spot changes or confuses the meaning. The test splits this error into two forms, misplaced and dangling, and both are repaired by lining the phrase up with the right noun.

Misplaced Modifiers

A misplaced modifier sits closer to the wrong noun than the one it is meant to describe. The sentence still has a clear subject, but the modifier attaches to it instead of the intended word. The result reads as a claim the writer never meant to make.

Broken: "Covered in dust, the librarian found the old manuscript." This wording says the librarian was covered in dust, not the manuscript. Fixed: "The librarian found the old manuscript covered in dust." Moving the modifier next to "manuscript" fixes the meaning.

Dangling Modifiers

A dangling modifier describes a word missing from the sentence entirely. The opening phrase has nothing correct to attach to, so the sentence needs a new subject. Rewriting the main clause is the usual fix, not moving the phrase.

Broken: "Walking through the exhibit, the paintings impressed the visitors." Paintings do not walk, so the phrase has no valid subject to describe. Fixed: "Walking through the exhibit, the visitors were impressed by the paintings." "Visitors" now follows right after the opening phrase and matches its action.

Check any sentence opening with a descriptive phrase followed by a comma. Read the phrase, then check whether the noun right after the comma is the one performing the action the phrase describes. When the noun and the action do not match, look for the choice rewriting the main clause rather than the phrase.

Parallel Structure in Lists and Comparisons

Parallel structure means every item in a list or comparison uses the same grammatical form. Mixing forms, such as a verb ending in "ing" next to a plain verb, breaks the pattern. The error is easiest to hear when you read the items back to back and drop the words sitting between them.

Parallel Lists

Each item in a series needs to match the grammatical form of the others, whether the form is a noun, a gerund, or an infinitive verb. Broken: "The program teaches students to write clearly, editing their work, and how to present ideas." The three items switch between an infinitive, a gerund, and a noun phrase. Fixed: "The program teaches students to write clearly, to edit their work, and to present ideas." All three items now use the same infinitive form.

Parallel Comparisons

A comparison built with words like "than" or "as...as" needs the same grammatical form on both sides. Broken: "Reading the report took longer than to summarize it." The comparison mixes a gerund with an infinitive. Fixed: "Reading the report took longer than summarizing it," which puts the gerund form on both sides. The table below summarizes what to check for parallel structure.

Pattern	Check	Example
List of three or more items	Same grammatical form on every item	"to write, to edit, to present"
Comparison with "than" or "as"	Same form on both sides of the comparison word	"reading... than summarizing"
Paired conjunctions ("either/or," "not only/but also")	Same form after each half of the pair	"either to call or to email"

Practice Sentence Structure Questions

Sentence Structure and Formation is one of three sub-skills under Conventions of Standard English. The other two, Punctuation and Usage, sit inside the same reporting category and count toward the same 38 to 43 percent band. Usage covers word choice and conciseness, so pair this guide with our ACT English style guide once you finish these drills. See our ACT English section guide for how the whole section is built and scored.

Our free drills split practice by error type, with instant explanations and no signup required. Work the Fragments & Run-ons drill for fragments, run-ons, and comma splices, then move to the Modifiers & Parallelism drill for misplaced modifiers and broken parallel structure. For full-length practice under real section timing, use our ACT exam page.