1) Grammar is a core component of language and defines its structure, consisting of phonetics, phonology, morphology, syntax, and semantics. Understanding a language's grammar is essential for both teaching and effectively communicating in that language. 2) As the rules that govern sentence structure and meaning, grammar is vital for language teaching. While native speakers can communicate without conscious knowledge of grammar, being able to analyze and explain grammar allows teachers to effectively instruct students and identify errors. 3) For English teachers, knowledge of both English grammar and the grammar of students' first languages is important. This helps teachers understand common mistakes students make and tailor instruction to their needs.