"What is Péter waiting for?"
Translation:Péter mit vár?
There is no indication that the object (what) should be pluralized. Also, it is more likely that they would say "Mire var...", since one waits on something. Using the accusative 't' on 'what' as an object would more closely translate to "What is Peter expecting?" rather than "What is Peter waiting for?"
No, there is no indication that the object should be pluralised, and so both versions (singular or plural) should be accepted; similarly with -t versus -re.
If you entered a version which is correct but is not yet accepted, please report it as an additional acceptable translation and help improve the course while it is in the beta stage! Thank you :)
I didn't catch "mikre var" because I was focusing on the accusative. This skill is early in the course, and I don't remember whether accusative vs. whatever the -ra/-re case is called was covered yet.
"Mikre" was in one of the multiple choice answers, which I didn't realize was also correct. It's mik (what in the plura) + re ("onto" ending). It's another way to express "what" as the object of an action (as opposed to "mit"). I don't know how many verbs allow this, so they might not always be interchangeable.
This is pretty much a specialty of vár. I suspect the usage with the -ra suffix came from German influence, because in German you literally wait "onto" something.
Most other verbs that call for direct objects are pretty straightforward with (only) using the -t suffix.
Often if you change the case of the object you chnage the meaning of the verb. Vár is one of the few where the meaning is the same for both.
Maybe it is the influence of the Ukrainian, as it uses "onto" after WAIT.
It is rather hard to learn further as the mobile version does not have any theoretical notes.
You should look for another source to teach you the grammar stuff. Duolingo is only really good for practicing translations, not for teaching.