Comma Splices, Run-ons, and Fragments: A Quick Fix Guide
Three sentence problems account for a large share of the marks lost in student writing and the confusion caused in professional writing. They are easy to spot once you know the pattern, and each has more than one repair. The Punctuation Fixer can clean the mechanical noise around them, but the sentence decision is yours.
Quick answer
A comma splice joins two complete sentences with only a comma. A run-on joins them with no punctuation at all. A fragment is a group of words that cannot stand alone as a sentence. Repair a splice with a full stop, a semicolon, or a conjunction; repair a run-on the same way; repair a fragment by adding the missing subject or verb, or by attaching it to the sentence it belongs to.
Comma splices
“The deadline moved to Friday, we need to finish the draft by Thursday.” Both halves can stand alone. A comma alone is not enough to join them in formal writing. The simplest repair is a full stop: “The deadline moved to Friday. We need to finish the draft by Thursday.”
A semicolon works when the two ideas are closely related: “The deadline moved to Friday; we need to finish the draft by Thursday.” A conjunction works when you want to name the relationship: “The deadline moved to Friday, so we need to finish the draft by Thursday.” Each repair changes the rhythm, so choose the one that matches the emphasis you want.
Run-ons
A run-on is the same problem without the comma: “The deadline moved to Friday we need to finish the draft by Thursday.” It is often caused by editing too quickly or by writing the way we speak. Read the sentence aloud and listen for the place where a full stop belongs. If you cannot hear it, find the second subject and verb.
Run-ons can be long, but length is not the test. “I went home” is a complete sentence; “I went home and I made tea and I sat down and I watched the news” is a run-on because it strings several independent clauses together without punctuation. A full stop, a semicolon, or a comma plus conjunction will fix it.
Fragments
A fragment is missing a subject, a verb, or a complete thought: “Because the train was late.” It has a subject and a verb, but “because” makes it dependent; it needs a main clause. “The train was late because of the signal failure” is complete. “Because the train was late, we missed the opening” is also complete.
Some fragments are deliberate. “Not yet.” “Always.” “A dangerous idea.” In advertising, fiction, and informal writing, a fragment can carry emphasis. In an academic essay or a formal report, the same fragment may be marked as an error. Match the convention of the document, not a rule you learned in isolation.
A quick checking method
Cover everything after the first possible sentence and ask whether it can stand alone. Then do the same for the next group of words. If a group cannot stand alone and is not deliberately styled, it is a fragment. If two groups that can stand alone are joined by only a comma, it is a splice.
Read the passage once for meaning before you fix the punctuation. A sentence that is grammatically correct but says the wrong thing is still wrong. For the spacing side of cleanup, read Why Pasted Text Has Double Spaces. For the general method, see How to Fix Double Spaces and Repeated Punctuation. You can clean repeated marks in the Punctuation Fixer before the read-through.
Common questions
What is the difference between a comma splice and a run-on?
A comma splice joins two independent clauses with only a comma. A run-on joins them with no punctuation at all. Both are sentence-boundary problems, and both can be repaired with a full stop, a semicolon, or a comma plus a coordinating conjunction.
Is a sentence fragment always wrong?
No. A fragment can be deliberate in fiction, advertising, and informal writing, where it creates emphasis or rhythm. In academic and formal writing, an unintentional fragment is usually marked as an error. Match the convention of the document and make the choice deliberately.
Can a punctuation tool fix a comma splice?
Not reliably. A comma splice is a grammar decision, not a repeated mark or a spacing problem. The tool can clean doubled spaces and repeated punctuation around it, but you still have to decide whether to use a full stop, a semicolon, or a conjunction.