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.
Do we need to study grammar to learn a language? The short answer is "no". Very many people in the world speak their own, native language without having studied its grammar. Children start to speak before they even know the word "grammar".
But if you are serious about learning a foreign language, the long answer is "yes, grammar can help you to learn a language more quickly and more efficiently." It's important to think of grammar as something that can help you, like a friend. When you understand the grammar (or system) of a language, you can understand many things yourself, without having to ask a teacher or look in a book.
A common contemporary definition of grammar is the underlying structure of a language that any native speaker of that language knows intuitively. The systematic description of the features of a language is also a grammar.
These features are the phonology (sound), morphology (system of word formation), syntax (patterns of word arrangement), and semantics (meaning). Depending on the grammarian’s approach, a grammar can be prescriptive (i.e., provide rules for correct usage), descriptive (i.e., describe how a language is actually used), or generative (i.e., provide instructions for the production of an infinite number of sentences in a language).
The traditional focus of inquiry has been on morphology and syntax, and for some contemporary linguists (and many traditional grammarians) this is the only proper domain of the subject.
Grammar improves your Listening and Speaking skills. Knowing Grammar helps us to communicate easier. For example, imagine you have a job interview, you need to talk about your past job experience. We apply the rules for Present Perfect tense. Because you talk about experience. When you meet a new person, you talk with them about yourself, you apply the present simple tense to talk about facts and regular habits. It is clear that the verbal clauses and adjective clauses help us enrich our sentences and add more information. Meanwhile, grammar rules help you to make your communication appear coherent and more logical.
Grammar improves your Reading and Writing skills Knowing basic grammar is important in order to be able to analyse and improve your language performance. Understanding the use of nouns, verbs, adverbs, adjectives, phrases or clauses helps you to write English better.
Also,knowing basic grammar is an important tool for readers to understand what they are reading. It is considered to be a handle of literature. News, stories or poems have a lot of long sentences.