✏️ Grammar · Punctuation & Mechanics
Comma Splices & Run-On Sentences
Why a comma cannot hold two sentences together, how to recognise a splice or a fused sentence in your own writing, the four ways to fix one, and the “however” trap that catches even strong writers.
Grammar › Punctuation & Mechanics › Comma Splices & Run-On Sentences
The rule in one sentenceA comma splice joins two complete sentences with only a comma and a run-on joins them with nothing at all — fix either one with a full stop, a semicolon, a comma plus a conjunction, or a subordinating word.
What’s the difference?
An independent clause is a group of words with its own subject and verb that could stand alone as a sentence: Idris Elba is British. He often plays Americans. The problem starts when you put two of them together without the right glue.
Use a comma alone and you have a comma splice: Idris Elba is British, he often plays Americans. Use nothing at all and you have a run-on (also called a fused sentence): Idris Elba is British he often plays Americans. Both are errors. The comma splice is far more common at B2 level because the comma feels like a join — but a comma separates; it does not connect.
🎬 Compare: “The Man Who Knew Too Much was released in 1956, it was a remake of a 1934 film” (comma splice) vs. “The Man Who Knew Too Much was released in 1956; it was a remake of a 1934 film” (correct).
One dot above the comma turns an error into a sentence. The semicolon is a join; the comma is not.
Length is not the issue. The Englishman Who Went Up a Hill But Came Down a Mountain is thirteen words long, yet it is a single grammatical unit: one subject (the Englishman) with a relative clause attached. There is no second independent clause, so there is nothing to splice. A run-on is about structure, not about word count.
How to fix it: the four repairs
1. Full stop
Of Mice and Men came out in 1992. It is based on a novel from the 1930s.
the simplest fix — two sentencesOf Mice and Men came out in 1992, it is based on a novel from the 1930s.
comma splice2. Semicolon
Idris Elba grew up in London; he has worked in Hollywood for years.
two closely linked ideas, one sentenceThose Magnificent Men in Their Flying Machines; or, How I Flew from London to Paris in 25 hours 11 minutes
the 1965 film’s full title uses a semicolon to add its second clause3. Comma + conjunction
Idris Elba is British, but he often plays Americans.
the comma is fine once a conjunction is thereIdris Elba is British, however he often plays Americans.
however is NOT a conjunction — see the trap below4. Subordination
Although The Man Who Knew Too Much is a remake, it is the better-known version.
one clause becomes dependent on the otherThe Man Who Knew Too Much is the better-known version because it was made second.
subordinator in the middle → usually no commaWhen to use which
Keep them separate (full stop / semicolon)
- Two equal, independent facts: Of Mice and Men was released in 1992. Idris Elba was not in it.
- Closely linked ideas you want in one breath: The title is long; the story is simple. — the semicolon says “these belong together”.
- Before a linking adverb: The Englishman Who Went Up a Hill But Came Down a Mountain is a comedy; however, it is a gentle one.
Join them (conjunction / subordination)
- Contrast: The title promises a mountain, but the film delivers a hill.
- Cause and result: Those Magnificent Men in Their Flying Machines is set in 1910, so the aeroplanes look terrifying.
- One idea depends on the other: Because Idris Elba has such a distinctive voice, he is often cast in voice roles.
Spot the pattern
The danger words are the ones that feel like conjunctions but are really adverbs. They can start a new sentence or follow a semicolon, but they cannot glue two clauses together after a comma.
Cannot join two sentences on their own
Can join two sentences
Visual guide
Teacher Teddy’s illustrated guide to this rule is being drawn — the full swipe-through infographic will appear here soon.
Common mistakes
❌ Idris Elba was born in London, he has worked in America for years.
✅ Idris Elba was born in London; he has worked in America for years. / Idris Elba was born in London, but he has worked in America for years.
Two independent clauses. The comma alone is a splice — upgrade it to a semicolon or add a conjunction.
❌ The Man Who Knew Too Much was released in 1956, however, it was a remake of a 1934 film.
✅ The Man Who Knew Too Much was released in 1956; however, it was a remake of a 1934 film.
The “however” trap. However is an adverb, not a conjunction, so it needs a semicolon (or full stop) before it and a comma after it.
❌ Of Mice and Men came out in 1992 it is based on a novel from the 1930s.
✅ Of Mice and Men came out in 1992. It is based on a novel from the 1930s.
A fused sentence — no punctuation at all between two clauses. Readers lose their place; a full stop gives it back.
British vs American note: The two varieties agree completely here. American style guides tend to insist on the comma after however (; however, it …); some British editors drop it in short sentences.
Exceptions & tricky cases
Three cases where a comma between clauses is acceptable — or only looks like a problem.
1. Very short, parallel clauses. In informal or literary writing, a comma is tolerated when the clauses are tiny and mirror each other: It’s not a hill, it’s a mountain. The villagers in The Englishman Who Went Up a Hill But Came Down a Mountain would approve — but in an exam essay, use a semicolon or a dash.
2. “However” in the middle of one clause. When however interrupts a single sentence rather than joining two, commas on both sides are correct: The Englishman, however, refused to give up. The test is whether the words before however form a complete sentence — here, The Englishman does not.
3. Long titles that are still one sentence. Those Magnificent Men in Their Flying Machines; or, How I Flew from London to Paris in 25 hours 11 minutes runs on for another clause — but notice the semicolon doing the join. And The Englishman Who Went Up a Hill But Came Down a Mountain has a but inside a relative clause, not between two sentences. Neither is a run-on.
Practice: splice, run-on or correct?
Eight quick questions. Choose the version you would write — you get the explanation either way.
1. Idris Elba is British ___ he often plays Americans.
A comma needs a conjunction to join two full clauses.
2. The film is long, ___ it never drags.
“However” is an adverb — after a comma it creates a splice. “But” is a real conjunction.
3. Of Mice and Men came out in 1992 ___ is based on a novel from the 1930s.
Two independent clauses → full stop (or semicolon). The comma version is a splice.
4. Which one is a comma splice?
The first has two complete sentences joined by a comma. The second uses a relative clause — one sentence, correctly punctuated.
5. I loved the film; ___, the ending felt rushed.
After a semicolon, a linking adverb like “however” (plus a comma) is the natural choice. “But” would normally follow a comma, not a semicolon.
6. The title is thirteen words long ___ it is still one sentence.
The “however” trap again: semicolon before, comma after.
7. Which one is a run-on (fused sentence)?
The second has two complete clauses with no punctuation between them. The first is a single clause with a participle phrase.
8. ___ Idris Elba grew up in London, he has spent much of his career in America.
“Although” subordinates the first clause, so the comma is correct. “However” cannot do that job.
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