ACT English · Sentence Structure
Sentence Boundaries on the ACT: Fragments, Run-Ons, and End Punctuation Explained
“Sentence boundary” questions make up about 20% of the ACT’s grammar section. In short, they’re a very important class of question to master. In fact, you will encounter more questions about sentence boundaries than any other type of grammar question. There are three different sub-types: fragments, run-ons, and sentence joining.
The good news is that the rules are pretty straightforward. Here’s the one thing you need to master for these questions: where does one independent clause end and another begin? If you can answer that, you won’t have much trouble. Remember that an independent clause is a grammatically complete thought on its own.
The ACT has one particular technique to camouflage grammar errors: wordiness. The most common trick is simply making the sample sentence so long and complex that the reader loses track of the preceding syntax. That means you do have to read more than just the underlined portion. If you don’t, you won’t have the right context to answer the question correctly.
Rules covered in this guide
| Rule | Named Method | Frequency |
|---|---|---|
| What is a comma splice and why is it always wrong on the ACT? | The Two-Sentence Test | Very High |
| How to identify and fix a run-on sentence on the ACT | The Pause-and-Check | Very High |
| How to identify and fix a sentence fragment on the ACT | The Stand-Alone Test | High |
| How to choose the right fix: period vs. semicolon vs. comma + FANBOYS | The Fix Decision Tree | Very High |
| Why “however” and “therefore” are not FANBOYS — the conjunctive adverb trap | The Conjunctive Adverb Rule | Medium |
| When an “-ing” phrase at the start of a sentence is NOT a fragment | The Subject Finder | Medium |
Rule 1
What Is a Comma Splice and Why Is It Always Wrong on the ACT?
Very High FrequencyA comma splice occurs when two independent clauses are joined by only a comma, with no coordinating conjunction. A comma alone is never sufficient to connect two complete sentences. This is one of the most frequently tested errors on the ACT English section, and it is always wrong — no exceptions.
Named Method
The Two-Sentence Test
Replace the comma in question with a period. Read each half as a standalone sentence. If both halves are grammatically complete sentences, you have a comma splice — and the comma must be replaced or reinforced. A comma alone between two complete sentences is always wrong on the ACT.
Three ways to fix a comma splice: (1) replace the comma with a period or semicolon, (2) keep the comma and add a FANBOYS conjunction after it, or (3) make one clause subordinate by adding a conjunction like “because,” “although,” or “since.” The answer choices will tell you which fix is being offered.
✗ Incorrect — comma splice
The committee approved the budget, the director announced the decision the following morning.
✓ Correct — semicolon fixes the splice
The committee approved the budget; the director announced the decision the following morning.
ACT-style practice question
The archaeologists had spent three field seasons excavating the site, the discovery of the bronze figurines in the final week made the entire project worthwhile.
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Rule 2
How to Identify and Fix a Run-On Sentence on the ACT
Very High FrequencyA run-on sentence occurs when two independent clauses are written back-to-back with no punctuation and no conjunction between them. Unlike a comma splice — which has a comma but no conjunction — a run-on has nothing at all between the clauses. Both are sentence boundary errors, and both are always wrong on the ACT.
Named Method
The Pause-and-Check
Read the sentence aloud and find the point where one complete thought ends and another begins. At that point, ask: is there any punctuation or conjunction? If there is nothing — no comma, no semicolon, no period, no FANBOYS word — you have a run-on. The fix is the same as for a comma splice: add a period, a semicolon, or a comma + FANBOYS conjunction at the boundary.
Run-ons on the ACT are often disguised by long sentences. The test will load the first clause with modifying phrases to push the boundary deep into the sentence, making the missing punctuation harder to spot. Always read to the end of the underlined portion and beyond before deciding.
✗ Incorrect — run-on, no punctuation
The report was submitted on time the board reviewed it within the week.
✓ Correct — period separates the clauses
The report was submitted on time. The board reviewed it within the week.
ACT-style practice question
The new transit line, which had been under construction for nearly a decade and had faced repeated funding shortfalls, finally opened to the public residents lined up before dawn to be among the first to ride it.
Rule 3
How to Identify and Fix a Sentence Fragment on the ACT
High FrequencyA sentence fragment is a group of words that is punctuated as a sentence but cannot stand alone as one. A fragment is missing a subject, a main verb, or both — or it begins with a subordinating conjunction that makes it dependent on another clause. On the ACT, fragments are tested both as errors to fix and as correctly punctuated constructions that students mistakenly try to change.
Named Method
The Stand-Alone Test
Isolate the clause in question and ask three questions in order: Does it have a subject? Does it have a main verb (not just a participle or infinitive)? Does it express a complete thought without depending on another clause? If the answer to any of these is no, it is a fragment.
Fragment fixes on the ACT usually involve one of three approaches: attaching the fragment to the preceding sentence with a comma, removing the subordinating conjunction that makes it dependent, or adding a missing subject or verb. The answer choices will always signal which type of fix is being tested.
✗ Incorrect — fragment, no main verb
The lead researcher, having devoted twelve years to the study of migratory patterns in the region.
✓ Correct — main verb added
The lead researcher, having devoted twelve years to the study of migratory patterns in the region, published her findings last spring.
ACT-style practice question
The renovation project transformed the abandoned warehouse into a thriving community arts center. Which now hosts over two hundred events each year and draws visitors from across the region.
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Rule 4
How to Choose the Right Fix: Period vs. Semicolon vs. Comma + FANBOYS
Very High FrequencyWhen all three fixes for a sentence boundary error appear as answer choices — a period, a semicolon, and a comma + FANBOYS conjunction — each is grammatically valid but not always equally correct. The ACT distinguishes between them based on the logical relationship between the two clauses and, most importantly, on whether the conjunction accurately represents that relationship. Choosing the wrong conjunction is wrong even if the punctuation is technically acceptable.
Named Method
The Fix Decision Tree
Step 1: Confirm both sides are independent clauses using the Two-Sentence Test. Step 2: Eliminate any answer with a conjunction whose meaning contradicts the relationship between the clauses — “but” implies contrast, “so” implies result, “and” implies addition. If the relationship is not contrast, eliminate “but.” If not a result, eliminate “so.”
Step 3: Between a period and a semicolon, both are usually acceptable — but on the ACT, one wrong answer will have an issue beyond just the period-vs-semicolon choice (a missing comma, an added conjunction, a changed word). The answer that is cleanly grammatical and preserves the original meaning is correct. When in doubt: the semicolon signals closely related ideas; the period signals more separation. Let the meaning of the sentence guide you.
✓ Correct — period (complete separation)
The funding was cut in March. The program continued on volunteer support alone.
✓ Also correct — semicolon (closely related)
The funding was cut in March; the program continued on volunteer support alone.
ACT-style practice question
The study confirmed that daily exercise improved cognitive performance in adults over sixty, but participants who maintained consistent sleep schedules showed even more significant gains across all measured categories.
Rule 5
Why “However” and “Therefore” Are Not FANBOYS — The Conjunctive Adverb Trap
Medium FrequencyConjunctive adverbs — words like “however,” “therefore,” “furthermore,” “nevertheless,” and “consequently” — look like they join clauses the way FANBOYS conjunctions do, but they do not. A comma before a conjunctive adverb between two independent clauses creates a comma splice. The correct punctuation before a conjunctive adverb joining two independent clauses is always a semicolon, never a comma alone.
Named Method
The Conjunctive Adverb Rule
When you see “however,” “therefore,” “furthermore,” “nevertheless,” “consequently,” or “moreover” between two clauses, the required pattern is always: independent clause + semicolon + conjunctive adverb + comma + independent clause. Missing the semicolon (replacing it with a comma) creates a comma splice. Missing the comma after the conjunctive adverb breaks the required pattern.
Memory check: FANBOYS use comma + conjunction. Conjunctive adverbs use semicolon + adverb + comma. These two patterns are never interchangeable on the ACT. If you see a conjunctive adverb with only a comma before it, it is wrong. Every time.
✗ Incorrect — comma before “however” = comma splice
The initial results were promising, however, the follow-up study failed to replicate them.
✓ Correct — semicolon before “however”
The initial results were promising; however, the follow-up study failed to replicate them.
ACT-style practice question
The city’s water conservation program had reduced residential usage by nearly thirty percent over four years, therefore the municipal water authority extended the program for an additional decade.
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Rule 6
When an “-ing” Phrase at the Start of a Sentence Is NOT a Fragment
Medium FrequencyA sentence that begins with an “-ing” word (a present participle) is not automatically a fragment. If the participial phrase is followed by a complete independent clause with its own subject and main verb, the sentence is grammatically correct. Students frequently misidentify these as fragments and incorrectly try to change them — making a correct sentence wrong.
Named Method
The Subject Finder
When a sentence begins with an “-ing” word or phrase, skip past the entire opening phrase and find the main subject and main verb of the sentence. If there is a clear subject and main verb after the participial phrase, the sentence is complete — it is not a fragment, and nothing needs to change.
Example: “Having analyzed the data carefully, the research team concluded that the hypothesis was correct.” Skip “Having analyzed the data carefully” — the main subject is “the research team” and the main verb is “concluded.” Complete sentence. The opening participial phrase modifies the subject and is separated from the main clause by a comma. This is correct and should not be changed.
✓ Correct — participial phrase + complete clause
Reviewing the manuscript one final time, the editor caught three errors that had survived earlier rounds of revision.
✗ Incorrect — true fragment, no main clause follows
Reviewing the manuscript one final time and catching three errors that had survived earlier rounds of revision.
ACT-style practice question
Drawing on decades of fieldwork across four continents, the anthropologist presented a theory of cultural exchange that challenged the dominant assumptions of her discipline.
Quick-Reference Summary: All 6 ACT Sentence Boundary Rules
| Rule | Named Method | Frequency |
|---|---|---|
| What is a comma splice and why is it always wrong on the ACT? | The Two-Sentence Test | Very High |
| How to identify and fix a run-on sentence on the ACT | The Pause-and-Check | Very High |
| How to identify and fix a sentence fragment on the ACT | The Stand-Alone Test | High |
| How to choose the right fix: period vs. semicolon vs. comma + FANBOYS | The Fix Decision Tree | Very High |
| Why “however” and “therefore” are not FANBOYS — the conjunctive adverb trap | The Conjunctive Adverb Rule | Medium |
| When an “-ing” phrase at the start of a sentence is NOT a fragment | The Subject Finder | Medium |
How to Approach Sentence Boundary Questions on Test Day
Tip 1
Always read past the underlined portion. Sentence boundary errors on the ACT are deliberately buried in long sentences. If you only read the underlined part, you will not see the error — the boundary between the two clauses is often 20 to 30 words away from the punctuation being tested. Read the entire sentence, both before and after the underline, before evaluating any answer choice.
Tip 2
Find the main subject and main verb of each clause before touching the answer choices. Once you know where one independent clause ends and another begins, the sentence boundary error becomes obvious. Strip out all the modifying phrases and prepositional phrases with a pencil if needed. What remains is the skeletal structure of the sentence, and the boundary will be clear.
Tip 3
When you see “however,” “therefore,” “furthermore,” or any other conjunctive adverb in an answer choice, immediately check what comes before it. If it is a comma, eliminate that answer — a comma before a conjunctive adverb joining two independent clauses is always a comma splice. The correct answer will have a semicolon before the adverb and a comma after it.
Common Questions About ACT Sentence Boundary Rules
In practice, the ACT will almost never ask you to choose between a period and a semicolon when both are grammatically correct, because the distinction is purely stylistic. What you are really being asked to decide is whether the answer choices have additional errors beyond the end punctuation itself — a missing comma, a wrong conjunction, a dependent clause where an independent one is needed.
Work through it this way: eliminate any answer with a conjunction that misrepresents the relationship between the clauses (“but” for addition, “so” for contrast). Eliminate any answer with a semicolon before a FANBOYS word or before a dependent clause. What remains will be the grammatically clean option. If a period and a semicolon are both still standing after elimination, choose the one that better reflects how closely the two ideas are related — semicolon for tight connection, period for more separation.
It is both a fragment and a dependent clause — those two things are not mutually exclusive. A dependent clause introduced by “which,” “although,” “because,” or “since” is always a fragment when it is punctuated as a standalone sentence. It is supposed to attach to the sentence before it (or after it). That is exactly the fix.
The practical test: if you put a period before “which” or “although” and read what follows as a standalone sentence, it will feel incomplete — because it is. “Although the funding was reduced” cannot end a sentence. “Which had been under construction for a decade” cannot begin one on its own. If the clause starts with one of these words and cannot stand alone, attach it to the adjacent independent clause with a comma.
No. A sentence never ends with a comma — not before “however,” not before “therefore,” not before anything. A comma is internal punctuation. What you are describing — a comma before “however” or “therefore” when those words connect two independent clauses — is a comma splice, and it is always wrong on the ACT.
The correct punctuation before “however” or “therefore” when they join two independent clauses is a semicolon, not a comma. The comma belongs after the conjunctive adverb: “The results were positive; however, the sample size was too small to draw conclusions.” Semicolon before. Comma after. That is the complete and only correct pattern.