The error, precisely
A comma splice is two complete sentences joined by a comma.
- Pollution is a serious problem, it affects everyone.
- Many students work part-time, they need the money.
- The chart shows a sharp rise, this continued until 2010.
Each of those is two sentences. Pollution is a serious problem is a sentence. It affects everyone is a sentence. A comma cannot hold them together — it is not strong enough, and in English that job belongs to a full stop, a semicolon, or a conjunction.
It is by a wide margin the most frequent punctuation error in IELTS essays, and it is especially common among candidates whose first language does allow clauses to be joined that way — which is a large number of languages.
Why it costs twice
This is the part that makes it worth prioritising over almost any other grammar point.
| Criterion | What it loses |
|---|---|
| Grammatical Range and Accuracy | Punctuation is explicitly inside this criterion. A splice is a punctuation error, and a repeated splice is a systematic one — which is what caps a band. |
| Coherence and Cohesion | This criterion asks whether the relationship between ideas is clear. A comma states no relationship at all. It puts two ideas next to each other and leaves the reader to guess how they connect. |
Look at the second row again, because it is the one people miss. Pollution is a serious problem, it affects everyone does not tell the reader why the second idea is there. Is it a consequence? Evidence? A restatement? The comma is silent.
Why 'write shorter sentences' is bad advice
The obvious repair is a full stop. Pollution is a serious problem. It affects everyone. That is now correct, and it has solved nothing.
Grammatical Range and Accuracy is scored on both halves of its name. An essay of short, simple, accurate sentences is accurate and demonstrates no range — and it is capped, reliably, around band 6. You have traded a punctuation error for a range problem, which is not obviously a better trade.
So the target is not a shorter sentence. It is the same long sentence, properly subordinated — which contains exactly the same two ideas, and adds the one thing that was missing: what they have to do with each other.
The five repairs
Take Pollution is a serious problem, it affects everyone. Five ways to fix it, in ascending order of what they earn you:
| Fix | Result | What it earns |
|---|---|---|
| Full stop | Pollution is a serious problem. It affects everyone. | Correct. No range. Use sparingly, for emphasis. |
| Semicolon | Pollution is a serious problem; it affects everyone. | Correct, and shows punctuation control. Still states no relationship. |
| Coordinator (and, but, so) | Pollution is a serious problem, and it affects everyone. | Correct, and names a weak relationship. Safe. |
| Subordinator (because, although, since, while) | Because it affects everyone, pollution is a serious problem. | A complex sentence. Names the relationship. This is what the criterion wants. |
| Relative clause (which, who) | Pollution, which affects everyone, is a serious problem. | A complex sentence, and the most economical of the five. |
The bottom two rows are the ones that raise a band, and notice that they are not longer than the splice. They are the same length with one word added — and that word is doing all the work, because it tells the reader how to read the second idea.
The words that name a relationship
This is the actual study list. A comma splice means one of these was missing, so learning them by what they mean rather than as a vocabulary list is the whole repair.
| The relationship | The words |
|---|---|
| Cause | because, since, as, given that |
| Result | so, therefore, consequently, which is why |
| Contrast | although, whereas, while, even though, despite the fact that |
| Condition | if, unless, provided that, in the event that |
| Addition | and, moreover, in addition, as well as |
| Description | which, who, whose, where |
When a long sentence is genuinely too long
Subordination can also be overdone. A sentence with four subordinate clauses is not four times as good — at some point the reader loses the thread, and Coherence penalises that just as it penalises the splice.
A practical rule: one main idea, at most two subordinate clauses. If you cannot say the sentence aloud in one breath, it has too much in it.
- Too much: Although many governments have introduced policies which are designed to reduce emissions, which have been rising for decades because of industrial growth that has not been regulated, the results, which vary considerably between countries, have been disappointing.
- Better: Many governments have introduced policies to reduce emissions, which have risen for decades under unregulated industrial growth. The results, however, have been disappointing, and they vary considerably between countries.
Two sentences, both complex, both readable. That is the target — variety, not length. An essay of uniformly long sentences is as monotonous as an essay of uniformly short ones, and range means moving between them.
How to fix it for good
- Learn to spot a complete sentence. Cover everything before the comma. Is what remains a sentence on its own? If yes, and the comma is the only thing joining them, it is a splice.
- Read your essay for commas specifically. Not for meaning — for commas. Check each one: is it holding two sentences together?
- When you find one, do not add a full stop. Add a because, an although, or a which. Ask what the relationship actually was, and name it.
- Watch however, therefore and moreover. They are adverbs. They need a full stop or a semicolon before them.
- Get it marked, because a systematic error is invisible from the inside — you produce it fluently, and it sounds right to you.
This is one of the highest-return corrections available in IELTS writing: a single grammar point, mechanical to check, that is damaging two criteria in nearly every essay that contains it.
Frequently asked questions
What is a comma splice?
Two complete sentences joined by a comma — 'Pollution is a serious problem, it affects everyone.' Each half is a full sentence, and a comma is not strong enough to hold them together; that job belongs to a full stop, a semicolon or a conjunction. It is the most common punctuation error in IELTS writing, especially among speakers of languages that do allow clauses to be joined that way.
Why does a run-on sentence cost so many marks?
Because it is scored twice. Punctuation sits inside Grammatical Range and Accuracy, so a splice is an error there — and a repeated splice is a systematic one, which caps a band. But it also damages Coherence and Cohesion, because a comma states no relationship at all: it puts two ideas side by side and leaves the reader to guess whether the second is a cause, a consequence or just a restatement.
Should I just write shorter sentences?
No — that trades one cap for another. Grammatical Range and Accuracy is scored on both halves of its name, so an essay of short, simple, accurate sentences shows no range and is reliably capped around band 6. The target is the same long sentence properly subordinated: 'Because it affects everyone, pollution is a serious problem' contains exactly the same ideas as the splice, plus the one thing that was missing.
Can I use 'however' and 'therefore' to join two sentences?
Not with a comma. They are adverbs, not conjunctions, so 'Pollution is serious, therefore we must act' is still a comma splice. They need a full stop or a semicolon before them: 'Pollution is serious. Therefore, we must act.' This catches out strong candidates constantly, precisely because the vocabulary itself is advanced.
How do I find comma splices in my own writing?
Read the essay for commas rather than for meaning, and check each one mechanically: cover everything before the comma, and ask whether what remains is a complete sentence on its own. If it is, and the comma is all that joins them, it is a splice. Then do not simply add a full stop — add a 'because', an 'although' or a 'which', and name the relationship you meant.