In linguistics we often focus on descriptive grammar. This means that we describe how language is actually used by people when speaking or writing. Think of it as someone listening to people speak at a cafe and taking notes on how they say things.
This contrasts with prescriptive grammar that states how a language “should” be spoken or written. This is what most of us think of when we hear the word “grammar.” Think of it as English class lessons when you were in grade school.
Here’s an example of the same basic sentence as you’d see it in descriptive and prescriptive grammars:
Descriptive: Who are you going to invite to the party? (This is what people actually say)
Prescriptive: Whom are you going to invite to the party? (This is what people “should” say)
random person:oh ma gawd guys, lok at dat man, dat shit crazy
me:the art of grammar, young padawan, is a diffcult and long road. You must be devoted. For it is your destiny.
linguistics:omg did they just monophthongize the nucleus of that first person possessive pronoun and shift their voiced interdental fricative to an alveolar stop reflecting it in the orthography and then delete the copula of an adjectival predicate that's so awesome desu~