This what frustrates me about grammar fanatics. Clearly language changes and evolves, and grammar is defined as standard usage, not some rule formed 50 years ago that no one follows in common speech. In short, grammar is fundamentally descriptive, not proscriptive.
It's both. Language changes, but at any particular time, there's some consensus about what's correct and incorrect. You can ignore it if you want, but people will think you're ignorant.
There may be an elite consensus, but more often than not that can differ from what is actually being spoken.
For example, in linguistics studies AAVE (or “Ebonics”) is considered like other dialects to basically have a grammar of its own that is internally consistent, and it is the first English of a fairly large population.
That’s before we get into which English is the “correct” one; there’s British English, American English, etc. for the US and CANZUK, but there’s also Indian English, Singaporean English, Euro English, etc.
