Session 10  ·  Student Handout  ·  Phase 2 · Standard English Conventions

Sentence Boundaries

Every sentence boundary error on the SAT comes down to one question: are two independent clauses being joined correctly? Two clauses, four legal ways to join them, and one test to diagnose anything that goes wrong.

First SEC Session Standard English Conventions Session 10 · Phase 2
01 The Two-Clause Test
Why Sentence Boundaries Matter
About a third of all Standard English Conventions questions on the SAT test sentence boundaries — run-ons, comma splices, and fragments. They all have the same cause: two independent clauses not being joined correctly. The Two-Clause Test diagnoses the problem in any sentence before you read the choices.
The Two-Clause Test — Apply Before Reading Any Choice
  1. Find the main verb(s). How many main verbs are there? Each main verb is the core of a potential clause.
  2. Identify each clause. Does each clause make a complete thought on its own? An independent clause (IC) can stand alone as a sentence. A dependent clause (DC) cannot.
  3. Label the structure: Is the sentence IC + IC, IC + DC, DC + IC, or just IC? Write it above the sentence.
  4. Apply the rule: IC + IC requires one of the four legal joiners. IC + DC or DC + IC is fine on its own. A DC with no IC attached is a fragment.
Independent vs. Dependent — The One-Sentence Test
Ask: "Can this clause stand alone as a complete sentence?" If yes → Independent Clause (IC). If no → Dependent Clause (DC).

IC: "The results were significant." ✓ Stands alone.
DC: "Although the results were significant" ✗ Does not stand alone — you're waiting for more.
DC: "Which had been published in 2019" ✗ Relative clause — cannot stand alone.
02 The 4 Legal Joiners for IC + IC

When you have two independent clauses, exactly four structures are legal. Any other structure is a run-on or comma splice.

JoinerStructureExampleNotes
.  Period IC. IC. The study was inconclusive. Further research is needed. Always correct. Two separate sentences. Safest option on the SAT.
;  Semicolon IC; IC. The results were surprising; no one had predicted this outcome. Both sides must be independently valid sentences. Do NOT use a comma here.
, + FANBOYS  Coordinating conjunction IC, FANBOYS IC. The study was limited, but the findings were compelling.
For · And · Nor · But · Or · Yet · So		 Comma BEFORE the FANBOYS word — not after. Both sides must be ICs.
Sub. conjunction  Subordinating IC [sub.conj. DC].
or [Sub.conj. DC], IC.		 The results shifted after the team reanalysed the data.
Although funding was limited, the study proceeded.		 Creates a DC, so no additional joiner needed. Common: although, because, since, while, when, if, unless, as, after, before, until.
⚠ The Comma-Only Trap (Most Common Error)
A comma alone CANNOT join two independent clauses. This is the comma splice — the single most common sentence boundary error on the SAT.

Wrong: "The study was limited, the findings were compelling." (comma splice — two ICs joined by comma alone)
Right: "The study was limited, but the findings were compelling." OR "The study was limited; the findings were compelling."

The comma splice is particularly dangerous because many comma splices "sound right" when you read them aloud. The ear is not a reliable editor — only the Two-Clause Test is.
03 The 3 Error Types
Comma Splice
Two ICs joined by a comma alone — no coordinating conjunction.
✗ The results surprised us, we had expected failure. ✓ The results surprised us; we had expected failure.
Fix: replace comma with period, semicolon, or comma + FANBOYS.
Run-On
Two ICs with no punctuation or joiner between them at all.
✗ The data was unclear the researchers revised their model. ✓ The data was unclear. The researchers revised their model.
Fix: insert a period, semicolon, or comma + FANBOYS between the clauses.
Fragment
A dependent clause or partial clause presented as a complete sentence.
✗ Although the experiment was carefully designed. ✓ Although the experiment was carefully designed, the results were ambiguous.
Fix: attach it to an independent clause, or rewrite to remove the subordinating conjunction.
04 The Paired-Dash Rule
Paired Dashes — Parenthetical Insertion Rule
IC — [parenthetical insertion] — rest of IC.
Dashes come in pairs when they set off a parenthetical phrase in the middle of a sentence. The key test: remove the material between the dashes — what remains must be a grammatically complete sentence on its own.

✓ The experiment — which had taken three years to design — produced unexpected results.
Remove the dashes: "The experiment produced unexpected results." ✓ Complete sentence.

✗ The experiment — which had taken three years to design produced unexpected results.
Missing the closing dash — the structure is broken.

Also watch for: opening a dash but closing with a comma (or vice versa). Dashes must be paired with dashes, not mixed with commas.
✗ The scientist — a pioneer in the field, changed the way we understood immunity.
✓ The scientist — a pioneer in the field — changed the way we understood immunity.
05 Worked Examples
Worked Example 1 Comma splice · IC + IC structure
Standard English Conventions
Coral reefs support approximately 25% of all marine species ______ they cover less than 1% of the ocean floor.
Two-Clause Test:
Clause 1: "Coral reefs support approximately 25% of all marine species" → Can stand alone ✓ → IC
Clause 2: "they cover less than 1% of the ocean floor" → Can stand alone ✓ → IC
Structure: IC + IC → needs one of the 4 legal joiners.
Meaning: these two facts contrast (large ecological role despite small area) → joiner should signal contrast → yet / although / but

Which choice completes the text so that it conforms to Standard English conventions?

Aeven though,— "Even though" creates a DC, which would make Clause 2 dependent — but Clause 1 has no joiner at all then. Fragment error.
B, they— Comma only between two ICs = comma splice. The most common wrong answer on this type.
C, yet they✓ Comma + YET (FANBOYS) joins two ICs correctly and signals contrast.
D; even though they— Semicolon followed by "even though" creates DC after the semicolon — but semicolons must be followed by an IC.

Key: C is the only option that legally joins two independent clauses (comma + FANBOYS "yet"). B is the comma splice trap. D is a semicolon followed by a dependent clause — illegal. A creates a dependent clause but leaves the first clause without a joiner.

Worked Example 2 Paired-dash rule · Remove and test
Standard English Conventions
The surgeon — who had performed the procedure thousands of times ______ completed the operation in under two hours.
Remove-and-test:
An opening dash has already been used: "— who had performed the procedure thousands of times"
Remove the material between dashes: "The surgeon ___ completed the operation in under two hours."
This must be a complete sentence → it is one, once the parenthetical is removed ✓
Therefore: the blank must close the parenthetical with a second dash, not a comma or nothing.

Which choice completes the text so that it conforms to Standard English conventions?

A, completed— Closes with comma instead of dash. Mismatched — the opening was a dash.
Bcompleted— No closing punctuation at all. The parenthetical is never closed.
C— completed✓ Closes the parenthetical with a matching dash. Remove and test: "The surgeon completed the operation in under two hours." ✓ Complete sentence.
D; completed— Semicolon would create IC; IC — but "The surgeon" still needs "completed" after it. The structure breaks entirely.

Key: Whenever you see an opening dash in a sentence, the next punctuation mark that closes that parenthetical must be another dash. The comma (A) and semicolon (D) both mismatch. No punctuation (B) leaves the structure unclosed.

Session 10 — The Three Rules
Ready to practice?

14 questions — comma splices, run-ons, fragments, and dash rules. Guided questions ask you to label clauses before answering. Timed section includes semicolon traps and paired-dash closure questions.

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