Grammar is the study of the structure and rules that govern the composition of words, phrases, clauses, and sentences in a language. Its primary subject is the system of linguistic patterns and principles that ensure effective communication. It focuses on two main areas: Morphology - the study of word formation and structure, and Syntax - the study of how words combine to form sentences. A grammatical category is a linguistic classification that expresses distinctions within a language, helping define how words function and relate to each other in a sentence. Common grammatical categories include tense (past, present, future) and number (singular, plural).