Translation:I don't know what these plates are for.
Why do we use the genitive case of what here? As far as I can tell we're not talking about possession, quantifying an amount or negating anything in the sentence.
Edit: Never mind, I forgot you always have to use the genitive with для
i think using Doulingo is not a good way for learning Russian language because in despite many other easier languages, in this language, learning padezha (падеж) is necessary and prior to everything else. i'm not very good in Russian complex grammer and unfortunately, i'm going to be disappointed and nearly to give up. simultaneisly i'm learning Spanish using Doulingo and don't have same problem with it because Spanish is extremely easier. by the way, English is not as my native language. please help me with useful comments. thanks a lot.
Ive found it useful to read a book explaining russian grammar while I use Duolingo. Duolingo does help with grammar rules if you go to the website and read the lesson descriptions, but they dont offer the descriptions on the phone apps. I use Rosetta stone too, but I have difficulty with knowing what the russian words actually mean on there, for example I thought подруга meant sister before I looked it up.
I think Duolingo is not enough on its own; you need books too if you want to learn the language well. However, I think it is excellent for vocabulary drilling, for practice at reading the Cyrillic script, and for getting used to the sound of the language. You can get other resources to help you with the grammar.