Run-on sentences and comma splices

A run-on sentence forces two (or more) independent clauses together without a legal join. When nothing at all stands between the clauses, it is called a fused sentence; when only a comma stands between them, it is a comma splice. Both count as the same underlying error: clauses that deserve their own sentences have been run together.

Length has nothing to do with it. A sixty-word sentence with correct joins is grammatical, and an eight-word sentence can be a run-on. The test is structural: find the subject-verb cores, then check what connects them.

• The tide came in the boats returned to harbour.(fused — two clauses, no join → The tide came in, and the boats returned to harbour.)
• Night fell, and the generators hummed into life across the camp.(correctly joined — length is not the problem)

A comma splice joins two independent clauses with only a comma. The comma is a separator inside sentences — it cannot carry the weight of a boundary between them. Splices are probably the single most common punctuation error in IELTS essays, partly because several other languages punctuate exactly this way.

The test is mechanical: cover everything left of the comma and read the right side alone; then swap. If both halves stand as complete sentences, the comma between them is a splice and needs upgrading.

• The library extended its hours, students still queued for seats.(both halves are complete sentences → …extended its hours, but students still queued…)
• Enrolment doubled, the university hired forty new lecturers.(→ Enrolment doubled, so the university hired forty new lecturers.)
• After the merger was announced, staff began updating their CVs.(the first part is a dependent clause — this comma is correct)

Every run-on has the same four repairs. One: a full stop — split it into two sentences. Two: a semicolon, when the clauses are closely related and you want them in one breath. Three: a comma plus a coordinating conjunction (and, but, or, so). Four: a subordinator (because, although, when), which demotes one clause and states the logic between them.

For IELTS, the fourth fix is the profitable one. It doesn't just remove the error — it manufactures a complex sentence and names the relationship (cause, contrast, time) between your ideas, feeding both Grammatical Range and Coherence at once.

• The visa was refused. We postponed the trip.(fix 1: full stop)
• The visa was refused; we postponed the trip.(fix 2: semicolon)
• The visa was refused, so we postponed the trip.(fix 3: comma + coordinator)
• Because the visa was refused, we postponed the trip.(fix 4: subordinator — the IELTS-friendly repair)

However, therefore, moreover, consequently, and nevertheless are conjunctive adverbs, not conjunctions. They comment on the link between ideas, but they have no grammatical power to join clauses — so 'clause, however clause' is still a comma splice, with an adverb sitting on top of it.

The repair is fixed and worth memorising: semicolon (or full stop) before the adverb, comma after it. This trap catches even strong candidates, because however feels exactly like but; grammatically, it behaves like an ordinary adverb such as unfortunately.

• The trial succeeded, however the funding was cut.(→ The trial succeeded; however, the funding was cut.)
• The trial succeeded. However, the funding was cut.(full stop before, comma after)
• Prices fell; therefore, demand recovered.(semicolon before, comma after)

The Band 7 writing descriptor requires 'good control of grammar and punctuation but may make a few errors'. One splice in an essay reads as a slip; three or more read as a system, and systematic punctuation faults are precisely what hold otherwise strong essays at Band 6. Splices also tax Coherence and Cohesion, because the reader keeps re-parsing where one idea ends and the next begins.

Speaking is unaffected — there is no punctuation in speech — which makes run-ons a purely writing-side gate, and one of the cheapest band-points to claim: two minutes of targeted proofreading removes the entire error class.

• Tourism brings revenue, it also strains water supplies, governments must balance both.(chained splices — the classic Band 6 sentence)
• Tourism brings revenue, but it also strains water supplies, so governments must balance both.(same ideas, legally joined)