

“den” is a pronoun which does not make sense/does not fit the gender in this context.
So I changed it to “denn” which is an adverb which in this case is hard to translate but boils down to something like “at all” in this sentence. Eg do you learn grammar there at all.
If you want to use a pronoun with the correct gender then it would be “die” (female) which is technically correct but sounds weird in this case.
So you could simply leave it out which would sound more natural.
Ja, definitiv: https://www.duden.de/rechtschreibung/Grammatik
Aber ich denke mit dem Beispiel “die Duolingo” hast du etwas anderes verstanden?
Trotzdem wäre es auch dort “die Duolingo (App)”. Was natürlich so keiner sagt, stimmt schon. 😁