Boundaries and Sentence Structure

Learning Objectives

• Distinguish between independent clauses (complete sentences) and dependent clauses (incomplete thoughts requiring a main clause)
• Identify and correct the three major sentence-boundary errors: run-on sentences (fused clauses), comma splices (clauses joined only by a comma), and fragments (incomplete clauses presented as sentences)
• Apply the four fixes for run-on sentences and comma splices: period, semicolon, comma + coordinating conjunction (FANBOYS), or subordinating conjunction
• Recognize and correctly punctuate appositives (renaming phrases), relative clauses, essential clauses, and nonessential clauses with commas, dashes, or parentheses
• Use introductory elements correctly: follow an introductory word, phrase, or clause with a comma before the main clause
• Apply colon rules: a colon follows a complete independent clause and introduces an explanation, list, or elaboration
• Apply semicolon rules: a semicolon connects two independent clauses without a conjunction
• Place modifiers correctly to avoid dangling modifiers and misplaced modifiers that create ambiguity
• Use sentence combining and sentence separation to improve clarity and achieve the appropriate relationship between ideas

High-Yield Concepts

Study Strategy

Begin with independent clauses vs. dependent clauses, since this distinction is the foundation for every other topic in the unit. A student who can quickly identify whether a clause is independent or dependent can immediately apply the correct punctuation rule in most boundary questions.

After this foundation, study the three error types (run-on, comma splice, fragment) as a cluster. These errors and their fixes are the most frequently tested topics in the unit. Practice identifying which error type is present, then apply the appropriate fix.

Then work through the clause and phrase types: appositives, relative clauses, essential vs. nonessential clauses, and introductory elements. These all require applying comma rules based on whether the element is essential or nonessential to the sentence's meaning.

Study colon usage and semicolon usage as a pair, since both connect independent clauses in distinct ways and are frequently confused. Then work through modifier placement (dangling modifiers, misplaced modifiers) as a separate cluster.

Common Mistakes

Using a comma alone to join two independent clauses. The comma splice is one of the most common errors on the SAT and in student writing. A comma is not sufficient to join two independent clauses; a coordinating conjunction (FANBOYS) must follow the comma, or the comma must be replaced with a period or semicolon.

Treating relative clauses as fragments. A clause beginning with "which," "that," "who," or "whose" is a relative clause, not a fragment, when it is attached to a main clause. However, a relative clause presented alone without a main clause IS a fragment. Verify that every relative clause is attached to an independent clause.

Using a colon after a fragment. Colons must follow complete independent clauses. "The findings: included three key results" is wrong because "The findings" is not an independent clause. "The study yielded three key results: strength, speed, and endurance" is correct because the colon follows a complete clause.

Confusing essential and nonessential clauses. Essential (restrictive) clauses define which specific thing is being discussed and cannot be removed without changing meaning; they take no commas. Nonessential (non-restrictive) clauses add extra information about something already identified; they require commas. The test: can you remove the clause without changing the sentence's meaning? If yes, it is nonessential; use commas.

Placing a semicolon before a dependent clause. A semicolon connects two independent clauses. If either side is a dependent clause, a semicolon is incorrect. Use a comma for dependent clauses following independent clauses.

Exam Tips

For essential vs. nonessential clause questions, use the "which/that" rule as a starting point: "that" clauses are typically essential (no commas); "which" clauses are typically nonessential (commas). This is not absolute, but it holds in the vast majority of SAT questions.

For modifier placement, identify the modifying phrase (especially introductory participial phrases: "Running through the park, ...") and check whether the subject of the main clause immediately follows the phrase. The noun immediately after an introductory phrase is what the phrase is understood to modify. If the noun after the phrase is not what the modifier logically applies to, the modifier is dangling and the sentence structure needs to change.