In the Middle Ages, grammar meant the study of Latin, because Latin was the language ofchoice for educated people. In fact, grammar was so closely associated with Latin that the word was also used to refer to any kind of learning. (You may have heard people from earlier generations your grandparents, perhaps
talk about their grammar school, not their elementary school. The term grammar school is a leftover from the old days. The very old days.)

    However, these day grammar is a study of  language, specifically, how words are put together to create meaning. Because of all of those obsessive English teachers and their rules, grammar also means a set of standard that you have to follow in order to speak and called usage, as in standard and non-standard  usage. Standard usage is the one that earns an A grade. It is the commonly accepted, correct patterns of speech and writing that mark an  educated person in our society, You'l l find standard usage in government documents, in newspapers and magazines, and in textbooks. Non-standard usage draws red ink from a teacher's pen faster than bullet cuts through butter. It includes slang, dialect, and just plain bad grammar.