Grammar is the system of a language. People sometimes describe grammar as the "rules" of a language; but in fact no language has rules*. If we use the word "rules", we suggest that somebody created the rules first and then spoke the language, like a new game. But languages did not start like that. Languages started by people making sounds which evolved into words, phrases and sentences. No commonly-spoken language is fixed. All languages change over time. What we call "grammar" is simply a reflection of a language at a particular time.