What counts as a run-on
A run-on is two independent clauses (each one a complete sentence with its own subject and verb) stuck together without a legal boundary between them. The SAT tests it in two flavors, and they are really the same error wearing different masks.
A fused sentence has nothing at all between the two clauses, not even a comma:
The greenhouse lost power overnight the seedlings survived anyway.
A comma splice puts a lone comma where a stronger mark is required:
The greenhouse lost power overnight, the seedlings survived anyway.
Both are wrong for the same reason. Each half is a full sentence, and a comma is not strong enough to hold two full sentences together. Nothing at all is obviously worse. The comma splice is the version most students under-detect, because a comma looks like it is doing a job. It is not. Treat the fused sentence and the comma splice as one problem with one test, and this whole question family collapses into a single move.
The Period Swap check
Here is the whole method. Find the exact spot where the answer choices change the punctuation. Mentally drop a period at that spot. Then ask one question: is what sits on both sides a complete sentence on its own?
- Complete on both sides: only a period, a semicolon, or a comma plus a coordinating conjunction is legal there. A lone comma is not. Nothing at all is not.
- Complete on only one side: you are not looking at a run-on. You are looking at a sentence fragment or a subordinate-clause question, which follows different rules.
That is the Period Swap, and it is the single highest-yield check in the whole Boundaries family. The reason it works is that run-ons, comma splices, and missing-comma errors are all the same underlying question (two complete sentences, one seam) with different wrong answers bolted on.
The trap is that the SAT hides the seam. On easy items the two clauses are short and the break is obvious. On hard items a long modifying phrase sits right where the boundary belongs, so the seam is not where your eye expects it:
The museum's new wing, funded by a decade of small gifts from local families, opens Friday visitors have already booked every tour.
Your eye wants to stop after "families," because that is where the commas are. The actual run-on is buried at "Friday visitors." Swap a period in there: "...opens Friday. Visitors have already booked every tour." Both halves are complete, so that seam needs a real boundary, and right now it has nothing. Fused sentence. The lesson: find the subject and verb of each clause instead of eyeballing length.
The four legal joins
Once the Period Swap confirms you have two complete sentences, you have exactly four legal ways to join them. Take this base pair:
The bridge closed for repairs. Commuters found new routes.
1. A period. Keep them as two separate sentences, exactly as written above. Always available, always safe.
2. A semicolon. A semicolon is a period in disguise, so both sides still have to be complete sentences.
The bridge closed for repairs; commuters found new routes.
3. A comma plus a coordinating conjunction (FANBOYS: for, and, nor, but, or, yet, so). The comma comes before the conjunction, and you pick the FANBOYS word that fits the logic.
The bridge closed for repairs, so commuters found new routes.
4. Subordination. Demote one clause to a dependent clause with a subordinating word (because, although, when, while, since, if, after). Now only one side is a full sentence, so there is no longer a two-sentence collision to punctuate.
When the bridge closed for repairs, commuters found new routes.
Those four cover essentially every correct run-on answer. A colon or a dash can join in narrow cases where the second clause explains the first, but treat those as specialty tools, not defaults. For the semicolon, see the semicolon rules guide, since the SAT loves to offer a semicolon before a fragment as a trap.
However is not a conjunction
This is the trap that quietly costs the most points: words like however, therefore, moreover, thus, consequently, nevertheless, and then are not coordinating conjunctions. They are conjunctive adverbs (transition words), and a comma in front of one does not fix a run-on.
The forecast looked clear, however the storm arrived by noon.
That is still a comma splice. "However" cannot do a conjunction's job, so the comma is illegal exactly as it would be with no word there at all. The legal repair is a period or a semicolon before the transition, with a comma after it:
The forecast looked clear; however, the storm arrived by noon.
The deeper lesson: only the seven FANBOYS words earn a comma to join two clauses. Every other connector you might reach for is a transition, and transitions ride behind a period or semicolon, not a comma. The full mechanics live in the however punctuation guide, and the logic of picking the right transition is its own skill covered in SAT transitions.
Traps to watch
- The hidden seam. A long descriptive phrase between the two verbs pushes the real boundary past where your eye stops. Find each subject and verb; do not judge by length.
- The FANBOYS comma that should not be there. The comma-plus-conjunction rule only fires when both sides are independent. "The team practiced hard and won the tournament" has one subject and two verbs, so it takes no comma. Add a subject ("and they won") and the comma becomes required. Whether a subject follows the conjunction is the whole test.
- The pronoun disguise. A second clause that starts with it, this, or they still counts as a full sentence. "The policy passed, it took effect in June" is a comma splice, not a continuation.
- The transition splice. Covered above: any comma splice where the culprit is however, therefore, then, or another conjunctive adverb.
The drill: find your run-on leak
Here is a self-test you can run today with your last practice section. Go through every Standard English Conventions question where the underline or the answer choices sit between two subject-verb pairs. On each one, run the Period Swap: drop a period at the seam and check both sides. Then look at whether the correct answer used a period, a semicolon, a comma-plus-FANBOYS, or subordination, and whether the wrong choices were fused sentences, comma splices, or transition splices.
If you solve these correctly when you slow down but miss them at full speed, that is not a knowledge gap, it is a behavior gap, the kind of leak that survives more studying. If you cannot spot the seam even when you slow down, drill the Period Swap until it is automatic. Either way, boundary questions are among the fastest points on the test to reclaim, which is why they anchor the SAT grammar rules toolkit.
Practice it now: the free simplesat grammar drill runs exactly these boundary questions and tells you which trap each wrong answer was, so you can see your run-on pattern instead of guessing. No signup, works as a guest: start the grammar drill.