ACT sentence structure: run-ons, fragments, modifiers and parallelism
ACT sentence structure questions ask whether a sentence is built correctly: no run-ons or comma splices, no fragments, modifiers next to the words they describe, and items in a list or pair written in the same form. ACT lists Sentence Structure and Formation under Conventions of Standard English, which is 38–43% of the English section. Start by finding the subject and main verb of each clause. If a choice leaves a clause without one, or joins two complete sentences with only a comma, it's wrong.
Where it shows up on the ACT
| Enhanced ACT | |
|---|---|
| Section | English |
| Length | 50 questions (40 scored) in 35 minutes, about 42 seconds each |
| Reporting category | Conventions of Standard English (Sentence Structure and Formation is one of its three skill groups) |
| Share of the section | 38–43% of English, about 15–17 of the 40 scored questions |
| In the Composite | Yes. The Composite averages English, Math and Reading. |
ACT publishes the share for the whole category, not for sentence structure alone. In the 40 scored English questions of ACT's 2025–26 practice test, we counted 4 sentence-boundary questions (run-ons, comma splices, fragments) and 2 on modifiers and parallel structure. Source: ACT, Preparing for the ACT 2025–2026, pp. 6–10: what each section covers and its reporting categories.
What the questions ask you to do
- Fix a comma splice or run-on: two complete sentences need a period, a semicolon, or a comma plus and, but, or, nor, for, so or yet.
- Fix a fragment: a group of words punctuated as a sentence that has no main verb, or that starts with because, although or which and never finishes.
- Put a modifier next to the word it describes, especially an -ing or -ed phrase at the start of a sentence.
- Keep items in a list or a pair in the same form: three verbs, three nouns or three -ing words.
- Choose the version where every clause still has its subject and verb after the change.
Sentence structure rules to know
Each row shows a pattern the ACT treats as correct, next to the error it replaces.
| Rule | Wrong | Right |
|---|---|---|
| Two sentences need a real joint | The ferry was late, we missed the bus. | The ferry was late, so we missed the bus. |
| A sentence needs a main verb | The mural, painted in 1978 by students. | The mural was painted in 1978 by students. |
| Because, although and which start a part, not a whole | Although the road was closed. | Although the road was closed, the bus got through. |
| An opening phrase describes the subject that follows it | Walking to school, the rain started. | Walking to school, Mia felt the rain start. |
| Lists and pairs match | She likes hiking, to swim and biking. | She likes hiking, swimming and biking. |
| Compare matching things | Her score was higher than her brother. | Her score was higher than her brother's. |
Common mistakes
Wrong answer choices are often built around these.
- Accepting a comma splice because the two sentences are short. "The ferry was late, we missed the bus" is still two sentences joined by a comma alone.
- Picking a period that leaves a fragment. A phrase like "Hoping to finish before dark." has no subject or main verb, so it can't stand alone.
- Treating which or because as harmless. Adding "which" to the start of a complete clause turns it into a piece that can't stand alone.
- Letting an opening -ing phrase describe the wrong noun. Whoever comes right after the comma is the one doing the action.
- Changing the form of only one item in a list. Check every item against the first one.
- Choosing the longest choice because it sounds formal. The shortest choice that keeps the sentence complete is often right.
3 example questions with answers
Written by Ashore in the style of the enhanced ACT. They aren't official ACT questions. Try each one before you open the answer.
Question 1 · Easy
The community garden opened in April with twelve raised beds, by July every bed had a waiting list of neighbors hoping to plant there.
Which choice makes the sentence most grammatically acceptable?
- ANO CHANGE
- Bbeds, and by July every bed
- Cbeds by July every bed
- Dbeds, by July, every bed
Show the answer
B. Both halves are complete sentences, so a comma alone can't join them. Adding and after the comma fixes the comma splice. With no mark at all the sentences run together, and adding a second comma around by July still leaves a comma splice.
Question 2 · Medium
Built from salvaged ship timbers, visitors are often surprised by the lighthouse because it looks so new.
Which choice makes the sentence most grammatically acceptable?
- FNO CHANGE
- GVisitors, built from salvaged ship timbers, are often surprised by the lighthouse
- HBuilt from salvaged ship timbers, the lighthouse often surprises visitors
- JBuilt from salvaged ship timbers, it is often surprising to visitors that the lighthouse is
Show the answer
H. The opening phrase describes what was built from ship timbers, which is the lighthouse, so the lighthouse has to come right after the comma. The original makes the visitors the ones built from timbers, the version with visitors first does the same, and the version starting with it attaches the phrase to a placeholder word and leaves "that the lighthouse is" unfinished.
Question 3 · Hard
To get ready for the regional contest, the debate team reviewed old topics, timing practice speeches, and they wrote new opening statements every weekend.
Which choice makes the sentence most grammatically acceptable?
- ANO CHANGE
- Breviewing old topics, timing practice speeches, and wrote
- Creviewed old topics, timed practice speeches, and they were writing
- Dreviewed old topics, timed practice speeches, and wrote
Show the answer
D. The team did three things, so all three verbs need the same form as reviewed: reviewed, timed and wrote. The original mixes reviewed, timing and a new clause with they. Starting with reviewing leaves the sentence without a main verb, and they were writing breaks the pattern again.
More ACT English guides
- Punctuation rules
- Grammar and usage rules
- Transitions
- Rhetorical skills
- ACT score calculator: questions right in each section to a 1–36 score and Composite.
- SAT vs ACT: which test fits you better.
- All ACT guides and free tools
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Practice test 1
Example
27
Composite
(28 + 26 + 27) ÷ 3
- English28
- Math26
- Reading27
- ScienceOptional, not in the Composite25