This paper described the concept of grammar, the knowledge and its usefulness in language use. It harped on the assumptions of the theory of competence and performance – the linguistic knowledge of a language and the use of this knowledge in concrete and different situations. Samples of the prescriptive grammatical rules of English used in sentences were given and types of sentences both according to structure and function also given to justify that it is grammar that governs all the activities related to language and language use. The paper noted that grammar is to a language, what a constitution is to a nation or that grammar does to a language what a football referee does to players in a football contest. And therefore, concludes that grammar is a mirror of language; it is a place to be and look at language.