Last updated July 07, 2026 · Reviewed by the AnvayaPrep team
Introduction
Sentence Structure is the foundational unit of the ACT English section, covering the rules that govern how complete, clear, and varied sentences are built. The 30 topics in this unit address every dimension of sentence formation: recognizing complete sentences versus fragments, identifying the boundaries between independent clauses, correcting run-on sentences and comma splices, deploying coordination and subordination to show logical relationships, and crafting varied sentence structures that engage readers without sacrificing grammatical accuracy.
On the ACT English section, sentence structure questions appear in the Conventions of Standard English category, which makes up roughly 55% of the 75-question test. Sentence Structure questions alone account for approximately 20-25% of English questions, meaning students encounter 15-20 sentence-level questions on every test. Because these questions are highly rule-based and follow predictable patterns, mastering sentence structure provides one of the clearest paths to score improvement on the English section.
The unit spans topics from the most foundational -- recognizing that a sentence needs a subject, a predicate, and a complete thought -- through the nuanced craft of combining short choppy sentences for variety or positioning sentence interruptions for emphasis. Topics like sentence fragments, run-on sentences, comma splices, independent and dependent clauses, and coordination and subordination form the structural core. Topics like sentence variety, sentence emphasis, sentence combining, and avoiding rambling sentences connect structure to style, bridging grammar conventions and rhetorical effectiveness.
Learning Objectives
- Identify complete sentences by confirming the presence of a subject, a predicate, and a complete thought
- Recognize sentence fragments created by dependent clauses, participial phrases, and missing subjects or verbs
- Identify and correct run-on sentences, including both fused sentences and comma splices
- Distinguish between independent and dependent clauses and apply appropriate punctuation to each relationship
- Use coordinating conjunctions (FANBOYS) and subordinating conjunctions to express logical relationships between clauses
- Recognize when a semicolon, comma, or period is required at a clause boundary
- Combine short sentences effectively using coordination, subordination, appositive phrases, and relative clauses
- Avoid choppy, rambling, or repetitive sentence patterns by varying sentence length and structure
- Position parenthetical information and sentence interruptions with correct punctuation
- Apply the isolation test to identify hidden fragments that sound complete in context
High-Yield Concepts
Sentence Fragments and Completeness
Sentence fragments are among the most frequently tested errors in this unit, appearing in 10-15% of sentence structure questions. A fragment lacks a subject, a verb, or a complete thought. The most common ACT fragment involves a dependent clause beginning with a subordinating conjunction (although, because, since, when, while, if) that is incorrectly separated from its independent clause by a period or semicolon. A second common type involves a relative pronoun clause (which, who, that) placed after a period when it should remain attached to the noun it modifies.
The isolation test is the most reliable identification strategy: read the suspected sentence alone, and ask whether it expresses a complete thought. If it leaves you waiting for more, it is a fragment. Length does not equal completeness -- "Although the research team worked tirelessly for three years" is a fragment despite its length.
Correction methods include attachment (joining the fragment to an adjacent independent clause), completion (adding missing elements), and removal (dropping the subordinating word).
When you see a period or semicolon followed by a subordinating conjunction (although, because, since, when, while) or a relative pronoun (which, that, who), immediately test whether the following structure can stand alone. If not, the punctuation is creating a fragment.
Run-On Sentences and Comma Splices
| Error Type | What It Is | Example | Fix |
|---|---|---|---|
| Fused sentence | Two independent clauses with no punctuation | "She studied she passed" | Period, semicolon, or comma + FANBOYS |
| Comma splice | Two independent clauses joined only by a comma | "She studied, she passed" | Add FANBOYS after comma, or use semicolon or period |
| Conjunctive adverb trap | Using "however," "therefore," or "moreover" with only a comma | "She studied, however she passed" | Semicolon before the adverb: "She studied; however, she passed" |
Comma splices are the more common ACT trap because the comma creates a false sense of correctness. The four correction methods are identical for both errors: (1) period, (2) semicolon, (3) comma + coordinating conjunction (FANBOYS: for, and, nor, but, or, yet, so), or (4) subordination. Conjunctive adverbs like "however," "therefore," and "moreover" are NOT coordinating conjunctions and require a semicolon before them when joining independent clauses.
Coordination and Subordination
Coordination joins independent clauses of equal weight using FANBOYS; subordination turns one clause into a dependent clause, making it secondary to the main clause. Each coordinating conjunction expresses a distinct logical relationship: "but" and "yet" signal contrast, "so" signals result, "and" signals addition, "for" signals reason. Choosing the wrong conjunction changes meaning even if grammar is technically preserved.
Subordinating conjunctions (because, although, when, since, if, unless, until, while, after, before, though) create dependent clauses that must attach to independent clauses. When a dependent clause opens a sentence, a comma separates it from the independent clause. When the independent clause comes first, no comma is typically needed.
Using a semicolon to separate a dependent clause from an independent clause creates a fragment error. Semicolons can only join two independent clauses. "The experiment succeeded; although the results surprised everyone" is incorrect because the second clause is dependent.
Sentence Variety and Combining
The ACT also tests whether students can recognize monotonous sentence patterns and select revisions that introduce structural variety. Simple sentences (one independent clause) create emphasis and clarity. Compound sentences (two independent clauses) show equal relationships. Complex sentences (one independent + one dependent clause) establish hierarchical relationships. Compound-complex sentences handle multiple connected ideas.
When combining short, choppy sentences, effective strategies include using coordinating or subordinating conjunctions, converting a sentence to a participial phrase or appositive, and incorporating relative clauses. The ACT rewards solutions that are both grammatically correct and concise, eliminating wordiness introduced by awkward combining.
Study Strategy
Begin with the foundational concepts: complete sentence recognition, independent clauses, and dependent clauses. These three topics establish the framework that every other topic in the unit builds upon. You cannot reliably correct fragments without understanding what makes a clause dependent, and you cannot correct run-ons without being able to identify two independent clauses.
Next, study sentence fragments, run-on sentences, and comma splices together as a cluster -- they are three facets of the same underlying concept (improper clause boundaries). Drill the FANBOYS mnemonic until it is automatic, and memorize the conjunctive adverb trap (however, therefore, moreover require semicolons, not commas).
After mastering the boundary concepts, move into coordination and subordination, which deepen your understanding of how clauses relate. Then study the structural variety topics (sentence combining, avoiding choppy sentences, sentence variety, sentence emphasis) as a group, since these topics ask you to make rhetorical as well as grammatical choices.
Topics such as adverb clauses, relative clauses, noun clauses, appositive phrases, introductory phrases, and parenthetical information are best studied once you are comfortable with clause boundaries, as they refine your ability to distinguish essential from nonessential elements and punctuate them correctly.
Reserve sentence interruption, sentence separation, sentence placement, and logical sentence flow for your final pass -- these topics lean toward organization and rhetorical skills, and they require the full sentence structure foundation to answer accurately.
Common Mistakes
- Treating length as an indicator of completeness; long dependent clauses are still fragments
- Thinking that any comma between two clauses is acceptable; a comma splice occurs when a comma joins two independent clauses without a coordinating conjunction
- Confusing conjunctive adverbs (however, therefore, moreover, consequently) with coordinating conjunctions; these words require semicolons before them, not just commas
- Using a semicolon before a dependent clause; semicolons can only separate two independent clauses
- Assuming that starting a sentence with "and" or "but" creates a fragment; sentences can begin with coordinating conjunctions if they contain a complete independent clause
- Selecting the longest answer choice as the best correction for a fragment; the ACT rewards concise, effective corrections
- Failing to read one sentence before and one sentence after an underlined portion; many sentence structure errors are only visible in context
- Overlooking relative pronoun fragments ("which," "who," "that" following a period) because they sound like they naturally continue the previous thought
- Combining sentences in a way that creates new errors such as dangling modifiers or misplaced phrases
- Confusing compound predicates with compound sentences; "She studied and passed" does not need a comma before "and"
Exam Tips
- When answer choices differ only in punctuation, map the clause boundaries first: identify each subject-verb pair, then decide whether each clause is independent or dependent
- Use the "period test": if you can replace the comma (or semicolon) with a period and get two complete sentences, you have two independent clauses that need proper connection
- Scan for FANBOYS immediately after a comma in underlined portions; a comma + FANBOYS joining two independent clauses is always correct
- Watch for conjunctive adverbs (however, therefore, moreover, furthermore, nevertheless) after commas -- this is the ACT's most common comma splice trap
- When you spot a subordinating conjunction (because, although, when, since, if) at the start of a sentence, check whether an independent clause follows; if the underlined portion is that entire structure, test whether it expresses a complete thought
- Sentence variety questions often appear with a "DELETE" or "OMIT" option or with choices that differ in sentence length; read the surrounding sentences to determine whether the passage suffers from monotony or over-complexity
- On fragment questions, eliminate "NO CHANGE" first if you have confirmed a fragment exists; then evaluate remaining options for conciseness and clarity
- Time allocation: sentence boundary questions (fragments, run-ons, comma splices) should take 15-20 seconds; sentence variety or combining questions may need 25-30 seconds to read context carefully
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