WHAT IS GRAMMAR?

Grammar

grammar (noun): the structure and system of a language, or of languages in general, usually considered to consist of syntax and morphology

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.
So think of grammar as something good, something positive, something that you can use to find your way - like a signpost or a map.
*Except invented languages like Esperanto. And if Esperanto were widely spoken, its rules would soon be very different.

https://www.englishclub.com/grammar/what.htm

GRAMMAR UNIVERSAL

universal grammar (UG) (noun): a theory in linguistics usually credited to Noam Chomsky that suggests that the ability to learn grammar is built into the human brain from birth regardless of language

In the 1960s, linguists became interested in a new theory about grammar, or the laws of language. The theory was popularized by an American linguist named Noam Chomsky who often focused on the effortless language learning of young children.
Chomsky didn’t believe that exposure to a language was enough for a young child to become efficient at understanding and producing a language. He believed that humans are born with an innate ability to learn languages. According to Chomsky’s theory, the basic structures of language are already encoded in the human brain at birth.
This “universal grammar theory” suggests that every language has some of the same laws. For example, every language has a way to ask a question or make something negative. In addition, every language has a way to identify gender or show that something happened in the past or present.
If the basic grammar laws are the same for all languages, a child needs only to follow the particular set of rules that his peers follow in order to understand and produce their native language. In other words, his environment determines which language he will use, but he is born with the tools to learn any language effectively.

Video: from Gene Searchinger's Human Language series. This clip discusses "Universal Grammar".

https://www.englishclub.com/grammar/theory/universal.htm