Asklemmy

I sont exactly know the meaning of these words,

The posh words? They aren't too important, but:

Modality is how a language allows you to talk about either reality or your attitude towards something (all languages do it). It's split into a bunch of categories, but the biggest ones are

• epistemic - you're talking about reality, things that are either true or false. For example, "the bar doesn't sell you lemonade". And
• deontic - you're talking about your attitude towards something; either "this is good/allowed/should be done" or "this is bad/disapproved/shouldn't be done". For example, "the bar should sell you lemonade."

In the meantime, a grammar for any given language can be either

• descriptive - it informs the reader how speakers of that language speak it; this is what Linguistics does. For example, "a lot of English speakers use «[noun] and me» as the subject." is a descriptive statement.
• prescriptive - it tells you how the author believes that you should be using that language. For example, "people shouldn't say «[noun] and me», they should say «[noun] and I»" would be a prescriptive statement.

You probably noticed that the underlying distinction between epistemic vs. deontic is the same as the one between descriptive and prescriptive. That was what "clicked" me years ago, that made me realise that this distinction is everywhere. And that, if we want clarity of reasoning, we should keep this distinction in mind.