Can grammar rules be useful?

Yes, they can. For example, if you don't hear (or read) some word or grammar pattern frequently, it may be hard to acquire a natural, intuitive knowledge of it.

Stop asking people to tell you grammar rules

Many learners have a strange habit. When somebody (e.g. a teacher) tells them the correct way to say something in English ("We say big red car.") or corrects their mistake ("You can't say red big car"), they like to ask "why?".
However, the question "why?" has no real answer. When asking the question, learners want to hear a grammar rule (e.g. "We say big red car because adjectives of size come before adjectives of color"). But the rule is not the reason why we don't say "red big car". The rule is only a description of native speakers' habits. It was invented by some linguist who simply noticed that native speakers never say "red big car" or "white small house".
In other words, it is not true that native speakers say "big red car" because they know the rule and follow it. It's the other way around. The size-color rule exists because native speakers say "big red car". Native speakers are the ones who create the language. Grammar rules only follow native speakers' habits.
I believe that it doesn't make much sense to ask the question "why is that sentence correct, and not the other one?". The only good answer to that question would be "Because native speakers say that sentence, and not the other one.". Instead of wondering "why?", simply learn the correct way. You don't have to care that a linguist wrote a rule for it. Follow native speakers, not grammar rules.

Graduation of my kids at EUROIDIOMAS LANGUAGE SCHOOL

left: the coordinator, Aylin, Angie, German, Marianne, Alison, Ivanna and of course Teacher Roger! "Good job !!!!

BOOK FAIR - PERFORMING ANNE FRANK'S DIARY AT CARLOS LISSON HIGH SCHOOL