Hey Charlie, How’s yer Grammer?
You need to understand this from the get-go. Speaking and writing are based on usage. The meaning of a word can change, depending on how it's used. Some of the time, the change begins as slang. Past generations thought being “cool” meant being emotionally distant. Being “hot” meant being angry or excitable. Today, “cool” means being “together” and “with it.” “Hot” means having sex appeal. If words stay with the same usage for a relatively long time, the dictionary will incorporate that into its list of definitions.
Words can also be changed into something completely new. “God be with you,” a kind of farewell when taking leave from friends, got scrunched together to become “goodbye.” The Pennsylvania localism, 'nebby,' originated from ‘neighborly.’ And “yeah,” easier to say than “yes,” is so common that it has landed in the dictionary.
The same variations can be seen in grammar, dating back to the seventeen hundreds when our Constitution was written. These so-called rules came about after the origin of the dictionary and the invention of the printing press. And you know what happens when something is put into print. Yup. It becomes gospel.
But grammar conventions (not rules) can and do change. The correct response to your mother's call of “Who is it?” from the kitchen when you came home from school should have been “It is I.” But kids (and the rest of us) feel this response sounds stuffy and self-important so “It's me,” has become the accepted form, at least conversationally. (To those eager to keep abreast of the so-called rule, it has to do with “is” being a linking verb which takes a predicate nominative instead of an object.)
Another example is the bowing to gender neutral writing. The sentence, “Each student should bring their books to school every day,” has a pronoun error, for the subject “each” is singular and “their” is plural, hence committing the fault of agreement. But instead of struggling with the awkward he/she, writers err in a different direction. And some day, this could be what today seen as error becoming an accepted second choice.
Meanwhile, Standard English Usage matters in social interaction, in jobs and in general interaction. This is not to denigrate community usage or the maintaining of one’s mother tongue. And it’s okay to be bi-lingual, whether it encompasses the cooing language a mother uses to a newborn baby or the shouting of a Little League at a tournament.