The SAT Writing section tests several essential grammar rules including subject-verb agreement, pronoun-antecedent agreement, verb tense consistency, modifier placement, parallel structure, punctuation usage, sentence completeness, diction, idiomatic expressions, and wordiness.

Subject-verb agreement ensures that subjects and verbs match in number, even when phrases come between them. A common error occurs with collective nouns or intervening phrases that can obscure the actual subject, leading to incorrect verb forms.

Parallel structure requires using the same grammatical form for all elements in a list or comparison. For example, 'She enjoys reading, swimming, and biking' maintains consistency by using the -ing form throughout, whereas 'She enjoys reading, to swim, and biking' breaks the pattern and creates an error.

A comma splice occurs when two independent clauses are incorrectly joined with only a comma. To avoid this error, use a semicolon to connect closely related independent clauses, or use a conjunction such as 'and' or 'but' along with proper punctuation.

A sentence fragment is missing a subject, a verb, or does not express a complete thought. Ensure every sentence has a subject and verb and forms a complete idea. Using appropriate conjunctions and punctuation helps connect related clauses properly.

Verb tenses must remain consistent within a sentence or group of related sentences. Shifting tenses without a clear time cue creates grammatical errors. Paying attention to time indicators in the passage helps determine which tense is appropriate.

Dangling modifiers occur when an introductory phrase does not clearly refer to the subject it is meant to modify. To fix this, place the modifier directly next to the word it describes so the meaning is immediately clear.

To improve your grammar skills, practice regularly with sample questions, review your mistakes to identify patterns, and read well-edited materials such as quality journalism or literature to internalize correct grammar usage.