If you are interested, Gracie, there are posts on English/grammar issues that I'm sure you will be studying this year.
They have been entitled "Setting the record straight on language issues # ..." and they have the tags, 'English" and "grammar".
I haven't yet explained fully, though I did touch on the issue when you asked, "Why do they teach us language that nobody uses?".
It's a very complicated issue and I will explain it as much as you need and want to understand. For now, I'll just tell you that I will give you the information on how English actually works, how it used differently in all the registers of English - formal and informal.
But I'll also tell you what it is that your teacher probably expects you to "learn" and repeat on your tests so that you can get a good grade, even though it might be an inaccurate explanation of real language.