What is the difference between a fragment and a run-on on the ACT?

A fragment is too little sentence: it lacks a subject, lacks a finite verb, or is made dependent by a subordinating word, so it cannot stand alone (for example, 'The hikers reaching the summit'). A run-on is too much: it joins two independent clauses incorrectly, either with no punctuation (a fused run-on) or with only a comma (a comma splice). Fragments are fixed by supplying a verb or removing the subordinator; run-ons are fixed by a period, a semicolon, a comma plus a coordinating conjunction, or subordination.

What are the four ways to fix a run-on or comma splice?

Use a period to make two sentences, a semicolon to link two closely related independent clauses, a comma plus a coordinating conjunction (the FANBOYS: for, and, nor, but, or, yet, so), or subordination to make one clause dependent with a word like because or when. Conjunctive adverbs such as however and therefore are not coordinating conjunctions, so a comma before them between independent clauses is still a comma splice; they need a semicolon.

How do I fix a dangling or misplaced modifier?

Put the modifier next to the word it describes. When a sentence opens with an introductory phrase and a comma (such as 'Walking home,'), the noun right after the comma must be the thing that phrase describes; if it is not, the modifier dangles, and the fix is to put the correct subject first. A misplaced modifier sits next to the wrong noun and should be moved beside the noun it logically describes. Watch limiting words like only and almost, which change meaning by position.

What does parallel structure require on the ACT?

Items doing the same job, in a list, a pair, or a comparison, must share the same grammatical form: all nouns, all -ing gerunds, all infinitives, or all clauses. Find the form set by the other items and make the odd one match. Correlative conjunctions (not only/but also, either/or, neither/nor, both/and) require parallel forms on each side, and comparisons with than or as must compare like with like, so compare a population to a population, not to a city.

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ACT English sentence structure and formation: complete overview of fragments, run-ons, joining clauses, modifiers, parallelism, and tense

A complete overview of sentence structure and formation on ACT English: complete sentences and fragments, run-ons and comma splices, joining clauses and conjunctions, modifier placement, parallel structure, and verb tense and consistency. The largest slice of the Conventions of Standard English category.

Generated by Claude Opus 4.811 min readACT-ENG-CSE-SSUpdated 2026-06-14

Reviewed by: AI editorial process; not yet individually human-reviewed

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The six sentence-structure skills
The thread through every skill: the complete sentence
How the items are tested
How to study sentence structure and formation
For the official exam materials

Sentence structure and formation is the largest slice of the Conventions of Standard English category, which is itself more than half the ACT English section, so it is where the most points sit. This site breaks the area into six dot points. This overview maps them, shows how they build on one definition (the complete sentence), and explains how to study them.

The six sentence-structure skills

Each skill is part of building and joining sentences correctly.

Complete sentences and fragments. What a clause needs (subject, finite verb, complete thought) and the three fragment types. See complete sentences and fragments.
Run-ons and comma splices. Two independent clauses joined wrongly, and the four standard fixes. See run-ons and comma splices.
Joining clauses and conjunctions. Coordinating, subordinating, and conjunctive-adverb connectors and their punctuation. See joining clauses and conjunctions.
Modifier placement. Dangling and misplaced modifiers and the introductory-phrase rule. See modifier placement.
Parallel structure. Matching forms in lists, correlative pairs, and comparisons. See parallel structure.
Verb tense and consistency. Keeping tense consistent with context and telling a wrong shift from a justified one. See verb tense and consistency.

The thread through every skill: the complete sentence

The organizing idea is that everything starts from the independent clause: a subject, a finite verb, and a complete thought. A fragment is a clause missing one of these. A run-on is two independent clauses joined incorrectly, so fixing it depends on recognizing where one clause ends and the next begins. Joining clauses is about connecting independent and dependent clauses with the right connector and punctuation. Modifiers must attach to the right part of the clause. Parallelism keeps coordinated parts in matching form. And tense consistency keeps the verbs across clauses in step. Learn the complete sentence cold, and the rest of the module becomes a set of checks you run on it.

How the items are tested

Underlined-portion questions: most of these skills appear as a single underlined portion with four versions, and you choose the one that forms a correct sentence.
Punctuation overlap: run-ons and joining clauses share rules with the punctuation module (semicolons, commas before conjunctions), so the same fix logic recurs.
Concision overlap: a grammatical option that is also more concise is usually better, linking sentence structure to Knowledge of Language.

How to study sentence structure and formation

Master the complete sentence so you can label any clause as independent, dependent, or a fragment.
Drill the four run-on fixes until choosing among period, semicolon, comma-plus-conjunction, and subordination is automatic.
Learn the three connector families and their punctuation (comma, none, semicolon-plus-comma).
Run the introductory-phrase check on every sentence that opens with a phrase and a comma.
Spot lists and pairs and match the form; read the context verbs to fix tense.

For the official exam materials

ACT, Inc. publishes the English test description and free official practice. See the description of the ACT English test and the test preparation page. Always study from the current official materials, because the question style is set by ACT.

Sources & how we know this

Description of the ACT English Test — ACT, Inc. (2025)
Preparing for the ACT Test — ACT, Inc. (2025)