ERROR 2: RUN-ON SENTENCES AND COMMA SPLICES
Definition
A run-on sentence occurs when two or more independent clauses are joined together without the correct punctuation or connecting words. There are two main types. The first is the fused sentence, in which two complete sentences are run together with no punctuation at all between them. The second is the comma splice, in which two independent clauses are joined only with a comma — which is not strong enough punctuation to connect two full sentences.
An independent clause is a group of words that contains a subject and a verb and expresses a complete thought. It can stand alone as a sentence. When two independent clauses need to be combined, the writer must use one of four correct methods: (1) a period to create two separate sentences, (2) a semicolon, (3) a coordinating conjunction (for, and, nor, but, or, yet, so) with a comma before it, or (4) a subordinating conjunction that makes one clause dependent.
Run-on sentences are among the most common errors in student writing. They occur because the writer can see the connection between two ideas and forgets to signal that connection with the correct grammar. A comma alone cannot join two sentences — this is perhaps the most important punctuation rule in English.
Rules
RULE: Two independent clauses cannot be joined by a comma alone. This is a comma splice.
RULE: Two independent clauses cannot be run together without any punctuation. This is a fused sentence.
RULE: Use a period, semicolon, or a comma + coordinating conjunction (FANBOYS: for, and, nor, but, or, yet, so) to join independent clauses.
RULE: Words like ‘however,’ ‘therefore,’ ‘moreover,’ ‘consequently,’ and ‘furthermore’ are conjunctive adverbs, NOT coordinating conjunctions. They cannot replace a comma to join clauses — they need a semicolon before them and a comma after.
Examples
✗ She was tired she went to bed early. [Fused]
✓ She was tired, so she went to bed early.
✗ The report was finished, it took three weeks to write. [Comma splice]
✓ The report was finished; it took three weeks to write.
✗ He studied hard however he still failed the test. [Fused with conjunctive adverb]
✓ He studied hard; however, he still failed the test.
✗ I love coffee, I drink it every morning. [Comma splice]
✓ I love coffee, and I drink it every morning.
✗ The movie was long it was also very entertaining. [Fused]
✓ The movie was long, but it was also very entertaining.
✗ She speaks three languages, moreover she is learning a fourth. [Comma + conjunctive adverb]
✓ She speaks three languages; moreover, she is learning a fourth.
✗ The children played outside all afternoon they came home exhausted. [Fused]
✓ The children played outside all afternoon and came home exhausted.
✗ This policy seems fair, nevertheless many people oppose it. [Comma splice]
✓ This policy seems fair; nevertheless, many people oppose it.
✗ He did not read the instructions, therefore the machine broke down. [Comma splice]
✓ He did not read the instructions; therefore, the machine broke down.
✗ The concert started at eight we arrived at seven to get good seats. [Fused]
✓ The concert started at eight, so we arrived at seven to get good seats.
Extended Dialogue
In this dialogue, college roommates Priya and James are editing each other’s essays before submission. James’s writing contains multiple run-on sentences.
Priya: James, can I give you some feedback on your introduction? I found a few grammar issues.
James: Sure, go ahead. What’s wrong?
Priya: This sentence: ‘Social media has changed the world it connects people across continents.’ Those are actually two separate sentences stuck together with nothing between them.
James: But I put a space between them?
[Note: A space is not punctuation. Two independent clauses need a period, semicolon, or coordinating conjunction — not just a space.]
Priya: A space isn’t enough. You need a period — ‘Social media has changed the world. It connects people across continents.’ Or you could write: ‘Social media has changed the world because it connects people across continents.’
James: What about this one: ‘The data shows a clear trend, however the researchers remain cautious.’ I put a comma before ‘however.’
[Note: ‘However’ is a conjunctive adverb, not a coordinating conjunction. A comma before it creates a comma splice. It requires a semicolon before it.]
Priya: That’s a comma splice. ‘However’ isn’t like ‘but’ or ‘yet’ — you can’t use just a comma before it. You need a semicolon: ‘The data shows a clear trend; however, the researchers remain cautious.’
James: Semicolon before, comma after?
Priya: Exactly. Same rule for ‘therefore,’ ‘moreover,’ ‘furthermore,’ ‘nevertheless’ — all of those.
James: Okay. Here I wrote: ‘The economy grew quickly, unemployment fell.’ That has a comma. Is that okay?
[Note: No — this is a comma splice. Two independent clauses joined only by a comma.]
Priya: Still a comma splice. You need to either add ‘and’ after the comma — ‘the economy grew quickly, and unemployment fell’ — or use a semicolon — ‘the economy grew quickly; unemployment fell.’
James: What’s the difference between those two options?
Priya: Stylistically, the semicolon emphasizes a close relationship between two equal ideas. The ‘and’ is simpler and more conversational. Both are correct.
James: I think I’ve been using commas everywhere because I’m nervous about periods making sentences too short.
Priya: Short, clean sentences are good! Don’t be afraid of periods. And when ideas are closely related, a semicolon is elegant. The one thing you can’t do is use a comma alone to glue two complete sentences together.
James: Can I always tell by looking for two verbs?
Priya: That’s a helpful start, but the key question is: do I have two groups of words that each have their own subject AND verb? If yes, you need real punctuation between them.
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