Who decides and changes the rules of grammar?

Answer 1

There are a few ways we can use to conceptualize grammar. If we take the idea the "rules of grammar" to mean the kind you follow in a language class to learn to write in proper academic style (prescriptive grammar), for example, then there are a number of people and groups who decide. These can range from language teachers to grammar book and dictionary writers to groups that officially oversee the language and decide what is "correct" and what is "incorrect" (such as the Académie française). On the other hand, the "rules of grammar" can also be thought of as the standards that competent speakers of a language must follow in order to be understood (descriptive grammar). Under this idea of grammar, the people who decide and change the rules of grammar are the speakers themselves, and those rules are constantly evolving and being redefined, even as you speak the language!