Sentence Structure

What you'll learn

• Recognize sentence fragments and understand why they are incomplete
• Identify and correct run-on sentences and comma splices
• Apply parallel structure to lists, comparisons, and paired constructions
• Identify and correct misplaced modifiers (the modifier is too far from what it describes) and dangling modifiers (the modifier has no logical subject)
• Choose the correct answer when multiple structural fixes are offered

Key concepts

A fragment lacks a subject, a verb, or a complete thought. A run-on sentence fuses two independent clauses without proper punctuation or a conjunction; a comma splice is a specific type of run-on that uses only a comma. Fix run-ons by adding a period, semicolon, or comma + coordinating conjunction. Parallelism requires that items in a series or elements being compared share the same grammatical form: 'She likes swimming, hiking, and *biking*' (not 'to bike'). On the ACT, parallel structure errors often hide in lists of three or more items or in correlative conjunctions (both/and, either/or, not only/but also). Modifier errors are another frequent trap. A misplaced modifier is too far from the word it modifies: '*Running quickly,* the finish line came into view' (the finish line is not running). Move the modifier next to the correct subject: '*Running quickly,* she saw the finish line.' A dangling modifier has no logical subject in the sentence at all. The ACT tests modifiers on roughly 3-5 questions per exam.

Pro tips

• If an answer choice creates a fragment by turning a clause into a participial phrase (e.g., replacing 'she ran' with 'running'), it is likely wrong.
• Two independent clauses joined by only a comma is ALWAYS wrong on the ACT. Look for a conjunction or stronger punctuation.
• For parallelism, underline each item in a list and check that they start with the same part of speech.