Run-On Sentences on the SAT
The Easiest Points You Might Be Missing
Master sentence structure, punctuation, and clarity with repeatable rules.
Guard the Sentence Core
Identify the subject and verb, then make sure punctuation does not split them or add extra ideas.
- Find the subject + verb first. That is the sentence core.
- Only add commas around extra information, never inside the core.
- Re-read the sentence without the modifier to test clarity.
Why Run-On Sentences Matters on the SAT
Some of the most frequently tested grammar questions on the SAT Reading and Writing section are also some of the most predictable, and run-on sentences fall squarely into that category. Once you learn to recognize them, these questions become near-automatic points, the kind of points that can push your score up without requiring any extra brainpower on test day.
The trick is that most students sort of know what a run-on sentence is, but they can't reliably spot one under pressure. This post will fix that. By the end, you'll know exactly what makes a sentence a run-on, the three legal ways to fix one, and the specific traps the SAT sets to catch students who aren't prepared. Let's turn your vague instinct into a sharp, test-ready skill.
A quick note on scope: this post zeroes in on fused sentences, run-ons where two independent clauses slam together with no punctuation at all. Comma splices are a closely related error, but they have enough nuance to deserve their own dedicated post. And if you're looking for guidance on when to coordinate versus subordinate your clauses, choosing between a semicolon and a "which" clause, for instance, that's covered separately as well.
What Is a Run-On Sentence?
A run-on sentence happens when two or more independent clauses, complete thoughts, each with its own subject and verb, are joined incorrectly. That's it. Despite what you might think, a run-on has nothing to do with length. A perfectly grammatical sentence can stretch across three lines. A ten-word sentence can be a run-on.
Run-ons come in two varieties:
- Fused sentence: Two independent clauses with no punctuation at all
between them.
Example: "The lab results were promising the team decided to continue the study." - Comma splice: Two independent clauses joined by only a comma, no
conjunction.
Example: "The lab results were promising, the team decided to continue the study."
Both are wrong. Both show up on the SAT. And both are fixable once you know the rules.
The Three Legal Ways to Join Independent Clauses
Every run-on question on the SAT boils down to the same core concept: there are only three correct ways to connect two independent clauses. Memorize these, and you'll have the tool you need for every version of this question.
- Period (or other end mark). Make them two separate sentences.
"The lab results were promising. The team decided to continue the study." - Semicolon. Use a semicolon when the two ideas are closely related and you
want to keep them in one sentence.
"The lab results were promising; the team decided to continue the study." - Comma + coordinating conjunction (FANBOYS). The seven coordinating
conjunctions are for, and, nor, but, or, yet, so. A comma plus one of these
words can join two independent clauses.
"The lab results were promising, so the team decided to continue the study."
That's the entire list. If an answer choice doesn't use one of these three methods to join two complete thoughts, it's wrong. On the SAT, this rule alone can help you eliminate answers in seconds.
The Conjunctive Adverb Trap
This is where the SAT gets sneaky. Words like however, therefore, moreover, consequently, and furthermore look and feel like conjunctions. They connect ideas logically. But grammatically, they are conjunctive adverbs, not coordinating conjunctions, and they cannot join two independent clauses with just a comma.
Wrong: "The experiment failed, however the researchers remained optimistic."
Right: "The experiment failed; however, the researchers remained optimistic."
Notice the fix: a semicolon before "however" and a comma after it. The SAT loves to offer comma-splice answers using conjunctive adverbs because they sound correct when you read quickly. Slow down at the boundary between clauses, check whether the connecting word is a true FANBOYS conjunction, and you'll avoid the trap every time.
SAT Strategy: The Boundary Check
Here is a simple, repeatable method for run-on questions on the SAT Reading and Writing section:
- Find the boundary. Read the underlined section and identify where one complete thought ends and another begins. Look for the spot where a new subject + verb combination starts.
- Check the connection. What's sitting at that boundary? Is it a period? A semicolon? A comma with a FANBOYS conjunction? Or is it just a comma, or nothing at all?
- Eliminate illegal joins. Cross out any answer choice that uses a comma alone between two independent clauses. Cross out any answer that fuses two clauses with no punctuation. Cross out any answer that uses a comma + conjunctive adverb without a semicolon.
- Choose the clearest survivor. Among the remaining choices, pick the one that preserves the original meaning and reads most naturally.
This process works for virtually every run-on question you'll see on the test. With practice, it becomes automatic.
Practice Run-On Sentences with SAT-Style Questions
Try these SAT-style questions. Each one tests your ability to spot and fix a run-on sentence. Start with the easier ones and work your way up.
Which choice best corrects the sentence?
Which choice correctly joins these ideas?
Which choice best corrects the punctuation in this sentence?
Which choice most effectively corrects the run-on sentence?
Key Takeaways for Run-On Sentences
- Run-on ≠ too long. A run-on is about how clauses are joined, not how many words are in the sentence. A short sentence can be a run-on. A long sentence can be perfectly correct.
- Memorize the three legal joins: period, semicolon, or comma + FANBOYS (for, and, nor, but, or, yet, so). If an answer choice doesn't use one of these to connect two independent clauses, eliminate it.
- Don't trust conjunctive adverbs. Words like "however," "therefore," and "moreover" are not coordinating conjunctions. They need a semicolon before them, not a comma. This is one of the SAT's favorite traps.
- Use the boundary check. Find where one complete thought ends and the next begins, check what's sitting at that boundary, and eliminate any answer that uses an illegal join. This method works every time.
Conclusion: The Core Rule for Run-On Sentences
Run-on sentences are one of the most predictable question types on the SAT Reading and Writing section, and that predictability is your advantage. The rules are simple, the traps are consistent, and once you've practiced spotting the boundary between independent clauses, these questions become some of the fastest and easiest points on the entire test.
Remember: Find the boundary, check the join, eliminate the fakes. Three steps, three legal methods, and a whole lot of easy points waiting for you on test day.
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