SAT Sentence Boundaries: Fix Fragments and Run-ons

Tell independent clauses from dependent clauses

An independent clause has a subject and finite verb and expresses a complete thought: “The glacier retreated.” A dependent clause also may have a subject and verb, but a marker such as “although,” “because,” “when,” or “which” makes it rely on another clause: “Although the glacier retreated.” A phrase may lack a subject-verb unit altogether: “During the unusually warm decade.”

Mark the core rather than every word. In “The map of several remote islands reveals a narrow channel,” the subject is “map,” and the finite verb is “reveals.” The prepositional phrase does not create another clause. In “The map reveals a channel that separates the islands,” “that separates the islands” is a dependent relative clause. Accurate classification prevents both unnecessary punctuation and fragments.

Repair the three major boundary errors

A fragment presents a dependent clause or phrase as if it were a complete sentence. Attach it to an independent clause or revise its structure. A run-on, or fused sentence, places two independent clauses together without a sufficient boundary. A comma splice joins those clauses with only a comma. These errors concern the relationship between complete units, not whether a sentence feels too long.

Two independent clauses can become separate sentences, join with a semicolon, or connect with a comma plus a coordinating conjunction. One can also become dependent: “The tide rose, and the path disappeared” can become “When the tide rose, the path disappeared.” The best repair depends on answer choices and meaning. Do not add or remove a conjunction without checking how it changes emphasis and logic.

Original worked example: classify before repairing

Consider: “Because the first recording contained heavy background noise. The linguists made a second recording indoors.” The first group has a subject and verb, but “because” makes it dependent, so the period leaves a fragment. Join it to the independent clause: “Because the first recording contained heavy background noise, the linguists made a second recording indoors.” The cause-and-result relationship is now complete.

Consider next: “The second recording was clearer, it preserved several faint consonants.” Both groups can stand alone. The comma creates a splice. A semicolon gives “The second recording was clearer; it preserved several faint consonants.” A period also works grammatically. A comma plus “and” is another legal structure. If choices include several legal boundaries, use context and the exact construction to determine the single best answer.

Handle transitions without creating a splice

Words such as “however,” “therefore,” and “moreover” can express a relationship, but they do not turn a comma into a sufficient boundary between independent clauses. Write “The sample was small; however, the pattern was consistent,” or use a period before “However.” By contrast, a comma plus a coordinating conjunction can directly join the clauses: “The sample was small, but the pattern was consistent.”

This distinction is frequently useful because answer choices may vary both punctuation and connector. First count independent clauses. Next identify the relationship. Finally choose a construction that does both jobs. If you begin with the transition’s meaning and ignore structure, a logically appealing comma splice may survive.

Common traps

Do not assume a sentence with multiple verbs contains multiple independent clauses; verbs may share a subject or appear in dependent clauses. Do not assume every “-ing” word supplies a finite verb. Avoid repairing a fragment by adding only a comma, or repairing a run-on by inserting a transition after a comma. Finally, do not split a subject from its verb because a long intervening phrase sounds like a natural pause.

Make clause recognition automatic

Practice classification separately from answer selection. Take 20 original word groups and label each independent clause, dependent clause, or phrase. Then combine them in more than one legal way and explain the meaning change. Move to fresh SAT-style questions, review errors by boundary type, and retest after a delay. Under time, the visible pattern—subject, finite verb, marker—should trigger the correct family of repairs.

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