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)