Who decides the rule of English grammar?

Answer 1

The users decide. But remember, there are several English registers, with different rules, as well as dialect forms that may be mutually unintelligible.

The poets and writers and orators of history have established the correct usages of the literary dialect, called Standard English. But media influence has elevated a chatty, less refined and more colloquial dialect to written status.

Answer 2

English grammar rules are based on tradition, common usage, and established authorities such as grammar guides and dictionaries. While there is no singular governing body that decides the rules, language evolves over time based on how people use it in everyday communication.