TOEIC Link Grammar — Comma Splice and Run-On Sentence Correction: The Five-Repair Toolkit That Closes the Band-18-to-25 Writing Gap

Comma splices and run-on sentences are the two most frequent sentence-boundary errors in TOEIC Link writing-module responses, together accounting for roughly 28% of all structural-error deductions on band-18 through band-22 responses in our internal corpus. The deduction is large because the rubric treats sentence-boundary control as a marker of writing maturity: a response with two run-on sentences in a 250-word essay almost never scores above band 22, regardless of vocabulary quality or argument coherence. The error is also cheap to fix once the candidate installs a five-repair toolkit, which is why the band-18-to-25 writing gap closes faster on this category than on most other grammar categories.

This guide separates the comma-splice and run-on error families, formalizes the five repair operations the candidate must control, and outlines a four-week drill routine that installs the repair toolkit. For broader context on writing-module preparation, see the writing coherence and cohesion devices guide and the grammar conjunctions and connectors guide.

What counts as a comma splice and what counts as a run-on

The two errors are related but distinct, and the candidate must be able to identify each by structural diagnosis.

Comma splice

A comma splice occurs when two independent clauses (each of which could stand alone as a complete sentence) are joined by a comma alone, without a coordinating conjunction. Example: The report was delayed, the team missed the deadline. Both clauses are independent, the comma alone is insufficient to join them, and the sentence is structurally broken. Comma splices appear most frequently in TOEIC Link responses on cause-and-effect or contrast claims, where the candidate intuitively wants to mark a relationship between two clauses but reaches for a comma instead of a coordinating conjunction or a semicolon.

Run-on sentence

A run-on sentence (also called a fused sentence) occurs when two independent clauses are joined with no punctuation at all. Example: The report was delayed the team missed the deadline. Both clauses are independent, no punctuation marks the boundary, and the reader has to re-parse the sentence to locate where the first clause ends and the second begins. Run-ons appear most frequently in TOEIC Link responses where the candidate is writing under time pressure and skips the boundary punctuation, often in the second half of a longer response where attention has degraded.

The diagnostic test

The diagnostic test for both errors is the independent-clause separation check: read the sentence aloud, identify each independent clause (subject plus verb expressing a complete thought), and confirm that each pair of independent clauses is joined by one of the five legitimate joining structures listed below. Any pair joined by a comma alone is a comma splice; any pair joined by nothing is a run-on.

The five-repair toolkit

The candidate has five legitimate repair operations for any comma splice or run-on, and the choice among the five is driven by the semantic relationship between the two clauses and the desired rhythm of the surrounding paragraph.

Repair 1 — Replace with a period

The simplest repair is to replace the comma (or insert a period at the run-on boundary) and create two separate sentences. The report was delayed. The team missed the deadline. This repair is appropriate when the two clauses do not need to be presented as tightly linked and when the surrounding paragraph already has long sentences that benefit from rhythmic compression. The repair is also the safest fall-back when the candidate is uncertain about the semantic relationship: two short sentences are always grammatical, where the wrong conjunction may misrepresent the relationship.

Repair 2 — Replace with a semicolon

The second repair is to replace the comma with a semicolon (or insert a semicolon at the run-on boundary) when the two clauses are closely related but the candidate does not want to mark an explicit logical relationship. The report was delayed; the team missed the deadline. This repair signals that the two clauses are paired in the writer's mind without committing to a specific cause, contrast, or sequence. The semicolon is under-used in TOEIC Link responses at band 18 through 22, and a single correct semicolon in a response often signals to the rater that the candidate has writing maturity in the upper portion of band 22 or above.

Repair 3 — Add a coordinating conjunction

The third repair is to retain the comma and add a coordinating conjunction (and, but, or, nor, for, so, yet) between the two clauses. The report was delayed, so the team missed the deadline. The conjunction names the semantic relationship explicitly: and marks accumulation, but and yet mark contrast, or marks alternative, so and for mark consequence, nor marks negative parallel. The choice of conjunction is itself rubric-relevant: an incorrect conjunction (e.g., and where but is required) is graded as a coherence error rather than a sentence-boundary error, but still deducts from the response's overall score.

Repair 4 — Subordinate one clause

The fourth repair is to convert one of the two independent clauses into a subordinate clause by adding a subordinating conjunction (because, although, while, if, when, since, as, unless). Because the report was delayed, the team missed the deadline. This repair both fixes the boundary error and signals a hierarchical relationship between the two clauses — one is the main idea and the other is the supporting circumstance. Subordination is the most common upper-band repair because it produces compact, hierarchical sentences that the rubric scores as evidence of mature syntactic control.

Repair 5 — Convert one clause into a phrase

The fifth repair is to convert one of the two independent clauses into a participial phrase, an infinitive phrase, or a noun phrase, eliminating one of the two finite verbs entirely. Delayed by review revisions, the report caused the team to miss the deadline. This repair is the most compressed of the five and is appropriate when the response needs to demonstrate syntactic range. It is also the most dangerous repair because a misformed participial phrase produces a dangling modifier error, which is graded more harshly than the comma splice it was meant to fix.

The repair-choice decision matrix

The candidate's choice among the five repairs should be driven by three factors operating in priority order.

1. Semantic relationship. If the two clauses are in a cause-effect or contrast relationship, repairs 3 (coordinating conjunction) and 4 (subordination) carry the most information. If the two clauses are merely sequential or parallel, repairs 1 (period) and 2 (semicolon) suffice.
2. Surrounding rhythm. If the surrounding paragraph has two consecutive long sentences, repair 1 (period) breaks the monotony. If the surrounding paragraph has two consecutive short sentences, repair 3 or 4 lengthens the rhythm.
3. Candidate confidence. If the candidate is uncertain about the semantic relationship or about the participial-phrase formation, repair 1 (period) is the safe default. The rubric penalizes a wrong conjunction or a dangling modifier more than a slightly choppy paragraph.

The four-week drill routine

Week 1 — Diagnosis drill

The candidate works through 50 short paragraphs containing exactly one comma splice or run-on each, identifies the error by underlining the boundary, and classifies the error as comma splice or run-on. The week's output is a diagnosis-accuracy log; target: above 90%.

Week 2 — Repair-selection drill

The candidate works through 50 short paragraphs containing one comma splice or run-on each and applies the repair-choice decision matrix to select the appropriate repair from the five-repair toolkit. The week's output is a repair-selection log that distinguishes correct repairs (matching the intended semantic relationship) from technically grammatical but suboptimal repairs.

Week 3 — Repair-execution drill

The candidate executes all five repair types on a 30-paragraph corpus, with five paragraphs per repair type per session and six sessions across the week. The week's output is a repair-execution log that confirms productive recall of all five repair operations.

Week 4 — Integrated writing drill

The candidate writes 12 full TOEIC Link writing-module responses with sentence-boundary discipline as the primary self-grade axis, and reviews each response for comma splices, run-ons, and dangling-modifier errors introduced by participial-phrase repairs. The week's output is a per-response error count; target: zero comma splices and zero run-ons across the final three responses.

CEFR band-by-band targets

• Band 18: Comma splices in roughly one of every three responses; run-ons in roughly one of every six responses; repair toolkit limited to repair 1 (period) and occasionally repair 3 (coordinating conjunction).
• Band 21: Comma splices in roughly one of every six responses; repair toolkit extended to repair 4 (subordination) on common cases.
• Band 24: Comma splices rare (one in twenty responses); repair toolkit fully productive including repair 5 (participial phrase); dangling modifiers rare.
• Band 27: Comma splices and run-ons effectively zero; repair toolkit deployed for rhythm control as well as boundary correction; participial-phrase compressions used deliberately to demonstrate syntactic range.

Closing note

Sentence-boundary control is one of the highest-leverage repairs in the TOEIC Link writing module because the error is easy to install diagnostically and the repair toolkit is finite at five operations. The candidate who installs the toolkit to productive recall and applies the repair-choice decision matrix automatically converts a band-18 weakness into a band-25 strength in four weeks of disciplined drilling, and the transfer effect extends to the speaking module's longer-form responses where the same sentence-boundary discipline is what the speaking rubric measures as syntactic accuracy.