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.
People often think of grammar as a matter of arbitrary pronouncements (defining ‘good’ and ‘bad’ language), usually negative ones like “There is no such word as ain’t” or “Never end a sentence with a preposition.” Linguists are not very interested in this sort of bossiness (sometimes called prescriptivism). For linguists, grammar is simply the collection of principles defining how to put together a sentence.
One sometimes hears people say that such-and-such a language ‘has no grammar’, but that is not true of any language. Every language has restrictions on how words must be arranged to construct a sentence. Such restrictions are principles of syntax. Every language has about as much syntax as any other language. For example, all languages have principles for constructing sentences that ask questions needing a yes or no answer, e.g. Can you hear me?, questions inviting some other kind of answer, e.g. What did you see?, sentences that express commands, e.g. Eat your potatoes!, and sentences that make assertions, e.g. Whales eat plankton.
The study of grammar all by itself won’t necessarily make you a better writer. But by gaining a clearer understanding of how our language works, you should also gain greater control over the way you shape words into sentences and sentences into paragraphs. In short, studying grammar may help you become a more effective writer.
Descriptive grammarians generally advise us not to be overly concerned with matters of correctness: language, they say, isn’t good or bad; it simply is. As the history of the glamorous word grammar demonstrates, the English language is a living system of communication, a continually evolving affair. Within a generation or two, words and phrases come into fashion and fall out again. Over centuries, word endings and entire sentence structures can change or disappear.
Prescriptive grammarians prefer giving practical advice about using language: straightforward rules to help us avoid making errors. The rules may be over-simplified at times, but they are meant to keep us out of trouble—the kind of trouble that may distract or even confuse our readers.