What Is a Run-on Sentence?
A run-on sentence occurs when two or more independent clauses are joined without the punctuation or conjunction needed to hold them together correctly. Each independent clause has its own subject and verb and can stand alone as a complete sentence. When you push two of them together without proper separation, the result is a run-on.
Here is a straightforward example:
The storm knocked out the power we lit candles and waited.
Both "The storm knocked out the power" and "we lit candles and waited" are complete thoughts. Fusing them without any punctuation or connecting word produces a run-on.
Run-ons confuse readers because they blur where one idea ends and the next begins. They are one of the most common errors in student writing, and they appear in professional writing just as often. Fortunately, they are also among the easiest errors to fix once you know what to look for.
If your writing also tends toward the opposite problem (clauses missing a subject or verb), see our guide on sentence fragments for comparison.
The Two Main Types of Run-on Sentences
Run-ons come in two forms. Knowing which type you are dealing with makes fixing it more straightforward.
1. The Fused Sentence
A fused sentence joins two independent clauses with nothing at all between them: no punctuation, no conjunction. The clauses simply collide.
She finished her report she sent it to her manager.
Both clauses are grammatically complete. Putting them side by side without any separator creates a fused sentence.
2. The Comma Splice
A comma splice is a run-on where the two independent clauses are joined by only a comma, which is not strong enough punctuation to do that job on its own.
She finished her report, she sent it to her manager.
The comma makes the sentence feel slightly more connected than a fused sentence, but the error is the same in principle: a comma alone cannot join two independent clauses. Many writers produce comma splices instinctively because a comma signals a pause, and a pause feels natural between two related thoughts. The feeling is understandable; the grammar is still wrong in formal writing.
Comma splices are common enough to have their own dedicated treatment. For a deeper look at that specific error, visit our comma splice guide.
How to Identify a Run-on Sentence
The key test is finding the independent clauses. Follow these steps when you suspect a run-on:
- Find the verb. Ask yourself who or what is performing the action. That gives you the subject. Together, subject and verb form the core of a clause.
- Count the independent clauses. If a sentence contains two separate subject-verb pairs that each express a complete thought, you have at least two independent clauses.
- Check what connects them. If the clauses are connected by nothing, or by only a comma, you likely have a run-on.
- Read the sentence aloud. A natural, noticeable shift in thought, with no grammatical signal to mark it, often points to a run-on.
Consider this sentence: "Marcus loves hiking he goes out every weekend." The verb "loves" pairs with "Marcus" (first clause). The verb "goes" pairs with "he" (second clause). Nothing connects them. That is a fused sentence.
Now consider: "Marcus loves hiking, he goes out every weekend." Same two clauses, now separated by a comma. That is a comma splice.
In both cases, the fix requires something stronger than a comma or nothing at all.
Four Ways to Fix a Run-on Sentence
There is no single "correct" fix. The best choice depends on how closely the two ideas are related and what rhythm you want in your writing. Here are the four standard methods, each illustrated with the same original run-on:
Run-on: The concert started late the crowd grew restless.
Method 1: Use a Period
Split the run-on into two separate sentences. This is the most decisive fix and works well when the two ideas are only loosely connected, or when shorter sentences suit the tone.
The concert started late. The crowd grew restless.
Two clean sentences. Simple and unambiguous.
Method 2: Use a Semicolon
A semicolon joins two independent clauses that are closely related in meaning. It signals a stronger connection than a period while still separating the clauses properly.
The concert started late; the crowd grew restless.
The semicolon implies a cause-and-effect relationship without stating it explicitly. Use a semicolon only when the two clauses are genuinely connected in thought, not just placed near each other.
You can also pair a semicolon with a conjunctive adverb such as therefore, however, or consequently for greater precision:
The concert started late; consequently, the crowd grew restless.
Note the comma after the conjunctive adverb; that is standard punctuation for this construction.
Method 3: Add a Coordinating Conjunction
A coordinating conjunction (one of the seven FANBOYS words: for, and, nor, but, or, yet, so) can join two independent clauses when placed between them with a comma before it. For a full rundown of how these words work, see our guide to coordinating conjunctions.
The concert started late, so the crowd grew restless.
The word so makes the causal relationship explicit. Choose the conjunction that best matches the logical relationship between the two clauses: but for contrast, and for addition, so for result, and so on.
Method 4: Use a Subordinating Conjunction
A subordinating conjunction turns one independent clause into a dependent clause. Common subordinating conjunctions include because, although, when, since, while, after, and unless. A dependent clause cannot stand alone; it must attach to an independent clause. This method works well when you want to show how the two ideas relate in time, cause, or condition.
Because the concert started late, the crowd grew restless.
Or: The crowd grew restless because the concert started late.
When the dependent clause comes first, follow it with a comma. When it comes second, the comma is usually omitted. Either word order works; choose the one that fits the emphasis you want.
When a Long Sentence Is Not a Run-on
One of the most persistent misconceptions about run-on sentences is that length causes them. It does not. A sentence can be long and still be grammatically correct. A sentence can be short and still be a run-on.
Consider this sentence:
Although the drive to the coast took nearly four hours and the hotel room was smaller than the photos suggested, everyone agreed the trip had been worth it.
That sentence is long, but it is not a run-on. It contains two dependent clauses ("Although the drive took nearly four hours" and "the hotel room was smaller than the photos suggested") attached to one independent clause ("everyone agreed the trip had been worth it"). Everything is properly connected.
Now compare that with a much shorter run-on:
I was tired I went to bed.
That is only five words, but it is a fused sentence: two independent clauses with nothing between them.
The rule is about structure, not length. Ask whether two complete thoughts are improperly joined. If they are, you have a run-on. If they are joined correctly with proper punctuation or a conjunction, the sentence can be as long as it needs to be.
Common Misconceptions About Run-on Sentences
"A sentence that goes on too long is a run-on."
Length is not the issue, as the section above explains. A run-on is a structural error, not a stylistic one. A writer can absolutely craft long, winding sentences that are grammatically sound. Run-ons are defined by the absence of correct joining mechanics between independent clauses.
"Any sentence with a comma is a comma splice."
Commas appear in correctly written sentences all the time: in lists, after introductory phrases, around parenthetical elements, and elsewhere. A comma becomes a splice only when it is the sole connector between two independent clauses. For a full overview of legitimate comma uses, see our guide to commas.
"I can always hear a run-on when I read aloud."
Reading aloud can help, but it is not foolproof. Well-phrased run-ons can sound natural, especially when the two clauses are closely related in meaning. Structural analysis, counting independent clauses and checking what connects them, is more reliable than sound alone.
"Fixing a run-on always means adding a comma."
A comma alone cannot fix most run-ons. Adding a comma to a fused sentence just converts it to a comma splice, which is a different form of the same error. A proper fix requires a period, a semicolon, or a comma paired with a coordinating conjunction.
Practice Examples with Corrections
Work through each example below. The original run-on is listed first, followed by one possible correction. In most cases several other corrections would be equally valid.
| Run-on | Type | One Correction |
|---|---|---|
| The dog escaped the yard the neighbors called us. | Fused sentence | The dog escaped the yard, so the neighbors called us. |
| I studied all night, I still failed the quiz. | Comma splice | I studied all night, but I still failed the quiz. |
| The café closes at nine we should leave soon. | Fused sentence | The café closes at nine; we should leave soon. |
| She speaks three languages, she learned them as a child. | Comma splice | She speaks three languages because she learned them as a child. |
| The report was incomplete it had several errors. | Fused sentence | The report was incomplete. It had several errors. |
| He apologized, she forgave him immediately. | Comma splice | He apologized, and she forgave him immediately. |
Notice that the corrections use different methods. No single fix is always best. The choice depends on the relationship between the ideas and the rhythm of the surrounding text.
If you want automated feedback on your own writing, the Rephrasely run-on sentence checker can flag potential fused sentences and comma splices in seconds.
Common Mistakes Writers Make
Even careful writers produce run-ons. Here are the patterns that tend to cause them.
Using a conjunctive adverb as if it were a coordinating conjunction
Words like however, therefore, consequently, and moreover are conjunctive adverbs. They can follow a semicolon and be followed by a comma, but they cannot join two independent clauses with only a comma before them.
Wrong: The plan failed, however we tried again.
Right: The plan failed; however, we tried again.
Joining clauses with "then"
The word then is an adverb, not a conjunction. It cannot join two independent clauses on its own.
Wrong: We finished dinner then we watched a film.
Right: We finished dinner, and then we watched a film.
Also right: We finished dinner. Then we watched a film.
Over-relying on commas for transitions
Writers who connect ideas fluidly in speech sometimes transfer that habit to writing by stringing clauses together with commas. Each clause needs a proper grammatical connector, not just a breath mark.
Forgetting the comma before a coordinating conjunction
When you fix a run-on with a coordinating conjunction, the comma goes before the conjunction, not after it.
Wrong: The meeting ran long but we covered everything.
Right: The meeting ran long, but we covered everything.
For a broader review of how commas interact with sentence structure, the Rephrasely grammar checker can help you catch punctuation errors alongside run-ons and other issues.
Frequently Asked Questions
What is the difference between a run-on sentence and a sentence fragment?
A run-on has too many independent clauses joined incorrectly. A sentence fragment has too few: it is missing a subject, a verb, or a complete thought. Both are structural errors, but they are opposites in terms of what is going wrong.
Is a comma splice always wrong?
In formal academic and professional writing, yes. In literary fiction and some personal essays, experienced writers occasionally use comma splices for stylistic effect, to create rapid pacing or to mirror the speed of thought. This works only when the writer clearly understands the rule. In most writing contexts, especially academic ones, treat the comma splice as an error to fix.
Can I fix every run-on with a semicolon?
Technically you can fix most fused sentences and comma splices with a semicolon, but it is not always the best choice. A semicolon signals that two closely related ideas share equal weight. If one idea is more important than the other, a subordinating conjunction often communicates the relationship more clearly. Overusing semicolons can also make writing feel monotonous.
Does a sentence need to be long to be a run-on?
No. As noted earlier, length has nothing to do with whether a sentence is a run-on. The error is structural: two independent clauses improperly joined. A five-word sentence can be a run-on; a fifty-word sentence can be perfectly correct.
How do I know which fix to use?
Ask what relationship the two ideas share. If they are equally important and closely connected, try a semicolon. If one causes or explains the other, a subordinating conjunction such as because or since works well. If they contrast, use but or yet. If you want maximum clarity and simplicity, split them into two sentences with a period.
What is a conjunctive adverb, and why does it matter?
Conjunctive adverbs are words like however, therefore, furthermore, meanwhile, and consequently. They look like they connect clauses, but they are adverbs modifying the clause they appear in. To correctly join two independent clauses using one, you need a semicolon before it and a comma after it: The data was incomplete; therefore, the conclusions remain tentative.