Is there any grammatical mistake in this sentence below?

Grammatical mistakes? No.

Answer to question: Yes.

But it's a senseless question, because they are related.

It would be like asking:

Are writing and spelling supposed to be taught simultaneously while learning English?

I would remove "the" from "the declensions" and I would remove "the" from "the cases.

Remove the "the" from cases, and possibly declensions, and put a comma before the "and," at least for British, and Canadian writing. Not for American.

I see nothing grammatically wrong with it.

That sentence looks good to me from a grammatical point of view. It is also good in general style.

Either you must write "the declensions and the cases", or "declensions and cases". "The" must exist for both or neither of those nouns, not just for one of them.