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SAT Reading and Writing: Boundaries (Medium)

Master boundary questions by spotting and correcting run-ons, fragments, and misplaced sentence boundaries, so every sentence in your writing is grammatically whole and logically clear.

Domain: Expression of Ideas & English Conventions Difficulty: Medium

By NUM8ERS Test Prep Team | Updated October 2025

What are Writing Boundaries?

In SAT writing, boundaries are the rules that define where one idea or sentence ends and another begins. Correct boundaries help your writing sound clear, logical, and whole. Boundary errors—like run-ons, comma splices, or fragments—break the rules that sentences must be complete and separated properly.

Key Types:
  • Run-on sentence: Two or more independent clauses joined incorrectly.
  • Comma splice: Joining two independent clauses with just a comma.
  • Sentence fragment: An incomplete thought punctuated as a complete sentence.
  • Fused sentence: Two independent clauses not joined by any punctuation or conjunction.
Core formula:
\(\text{Complete Sentence}\) \(\iff\) \(\text{Subject} + \text{Predicate} + \text{Complete Thought}\)

Top Tips for Boundaries Questions

  • Every complete sentence needs a subject and a predicate (verb + rest), and must express a complete thought.
  • Check for run-ons: two complete sentences joined by only a comma or no punctuation.
  • Don’t accept fragments: missing subject, verb, or complete idea.
  • To fix a run-on: use a period, semicolon, or coordinating conjunction (and, but, so, or).
  • Use the “read aloud” test—if you pause or get lost, check the sentence boundary.

Worked Example 1: Run-on Sentences

Incorrect: Koalas are marsupials they carry their young in pouches.

Which revision corrects the boundary error?

Correct: Koalas are marsupials. They carry their young in pouches.

✔ Use a period to separate the two independent clauses for clarity.

Worked Example 2: Comma Splice

Incorrect: The platypus is a mammal, it lays eggs.

Which revision fixes the boundary?

Correct: The platypus is a mammal, but it lays eggs.
or The platypus is a mammal; it lays eggs.
or The platypus is a mammal. It lays eggs.

✔ Join the sentences with a conjunction, semicolon, or period.

Worked Example 3: Sentence Fragment

Incorrect: Because the storm intensified.

Which revision makes it a complete sentence?

Correct: Because the storm intensified, the marathon was postponed.
or The marathon was postponed because the storm intensified.

✔ Add or connect a complete idea.

Quick Example: Fused Sentences

Incorrect: Owls are nocturnal they hunt at night.

How can you fix the fuse?

Correct: Owls are nocturnal. They hunt at night.
or Owls are nocturnal; they hunt at night.
or Owls are nocturnal, and they hunt at night.

Common Pitfalls and How to Avoid Them

  • Don’t split compound verbs or objects into separate fragments (e.g. “She ran. And jumped the hurdle.”)
  • Avoid “comma splices”: two full sentences joined by just a comma.
  • Don’t accept contractions, adverbs, or transitions as substitutes for full sentence boundaries.
  • Check that every answer is an independent clause unless it’s a proper dependent clause attached to a main one.

Key Takeaways

  • Every sentence boundary must be accurate for ideas to be clear, logical, and whole.
  • Run-ons and fragments can always be fixed by either separating, joining, or completing the thoughts.
  • Read sentences aloud—if it “sounds wrong,” check for a boundary error.
  • Look for full subject + verb + complete thought for a valid sentence.
  • Practice boundary recognition: fast, daily error-check drills.

Study & Practice Strategy

📚 Build Skill

  • Do daily boundary error-spotting drills.
  • Highlight sentence subjects, predicates, and punctuation marks.

🎯 Practice Wisely

  • Mix boundary practice with transitions, structure, and sense tasks.
  • Explain why each fix works—don’t just memorize corrections.

💡 Study Real SAT Edits

  • Model your edits on official SAT sentence boundary questions and explanations.

📖 Related SAT Skills

  • Form, Structure, and Sense
  • Transitions
  • Rhetorical Synthesis

🎓 NUM8ERS Boundaries Mastery

NUM8ERS Dubai’s signature approach trains students to spot and fix every boundary error with speed and confidence, using rapid practice, targeted feedback, and deep understanding of clause logic. Our students excel on the SAT by making every sentence complete and crystal clear.