Subject-verb agreement is a grammatical principle that ensures the subject and verb in a sentence match in number (singular or plural). In other words, a singular subject requires a singular verb, and a plural subject requires a plural verb. This agreement helps maintain clarity and coherence in writing. For example, "The dog barks" (singular subject "dog" with singular verb "barks") and "The dogs bark" (plural subject "dogs" with plural verb "bark"). Understanding and applying subject-verb agreement is crucial for effective communication and grammatically correct writing.