SAT Reading and Writing: Boundaries

Learn to spot and correct sentence boundaries—eliminate fragments, run-ons, and splices for clear, error-free writing and top SAT scores.

Domain: Expression of Ideas & English Conventions Difficulty: Advanced Unit: 10

By NUM8ERS SAT Prep Team | Updated October 2025

Boundaries Overview

Sentence boundaries are the rules for where one idea or sentence ends and another begins. Clear boundaries are essential for writing that makes sense and flows logically. Errors (like run-ons, comma splices, and fragments) break the basic rule: every sentence must be grammatically complete and properly separated from others.

Boundary Formula:

          \(\text{Correct Sentence} = \text{Subject} + \text{Predicate} + \text{Complete Thought}\)
        

Run-on: Two or more independent clauses fused without proper punctuation or conjunction.
Comma splice: Two independent clauses joined only by a comma.
Fragment: An incomplete thought punctuated as a sentence.

Top Tips: Boundaries

Every sentence needs a subject and predicate (verb + rest), and a complete thought.
For run-ons or splices, use a period, semicolon, or coordinating conjunction (and, but, so, or).
Fragments often miss a subject, verb, or full idea—don’t let them stand alone.
Read aloud: if you have to pause, check for boundaries.
Don’t join ideas with just a comma, and don’t break up compound ideas into fragments.

Worked Examples: Boundaries

Example 1: Run-On Correction

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

Correct: "Koalas are marsupials. They carry their young in pouches."
Use a period or semicolon to separate two independent clauses.

Example 2: Comma Splice

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

Correct: "The platypus is a mammal. It lays eggs." or "The platypus is a mammal; it lays eggs."
Don’t use just a comma between complete sentences.

Example 3: Fragment Fix

Incorrect: "Because the storm intensified."

Correct: "Because the storm intensified, the marathon was postponed."
Add a complete main clause to finish the idea.

Example 4: Fused Sentence

Incorrect: "Owls are nocturnal they hunt at night."

Correct: "Owls are nocturnal. They hunt at night."
Separate or connect ideas properly.

Example 5: Compound Part Fragment

Incorrect: "She ran. And jumped the hurdle."

Correct: "She ran and jumped the hurdle."
Don’t split compound predicates into sentence fragments.

Quick Example: Boundaries

Fragment: "Which was the reason for his delay."

Complete: "Traffic was heavy, which was the reason for his delay." (Joined with a complete main clause.)

Why? Every complete sentence needs a subject, a verb, and a clear, finished thought.

Common Pitfalls and How to Avoid Them

Splitting up compound verbs or objects ("walked. And smiled.").
Using a comma (without conjunction) to join sentences—true comma splices.
Accepting a phrase fragment as a full sentence—every statement needs a subject and a verb.
Letting transitions ("because," "which") hang alone without a main clause.
Confusing length with completeness—a long phrase isn't always a complete sentence!

Key Takeaways

Every sentence boundary must be accurate for ideas to be clear, logical, and whole.
Run-ons, fragments, and comma splices can always be fixed by separating, joining, or completing each thought.
Read sentences out loud—if it “sounds off,” check for boundary errors.
Look for a true subject + verb + complete idea for every sentence.
Practice recognizing errors; do quick daily boundary drills to build skill.

Study & Practice Strategy

Boundary Spotting Drills

Underline subjects/predicates in practice sentences. Check every clause for completeness.
Correct 3 real SAT questions a day focusing only on boundaries.

Official Resource Review

Study College Board or Khan Academy samples for boundary patterns and error fixes.

Related SAT Skills

Transitions
Form, Structure, and Sense
Sentence Combining

🎓 NUM8ERS Sentence Boundary Mastery

At NUM8ERS Dubai, students develop sentence boundary “radar”—catching run-ons, fragments, and splices with confidence for flawless writing on test day.