But "a buszon" could be either in, or on top of, the bus. The same goes for "hajón" (ship), "repülőn" (plane), "repülőgépen" (airplane), "vonaton" (train), "villamoson" (tram).
Because that is how we normally say it when we travel by those vehicles. So, in that case, we would have to say "a busz/vonat/etc. tetején" (on top of), to be specific.
But with cars, we travel in them. "Autóban". So, "az autón" definitely means "on the car".
Similarly in English, where are usually "on the bus", "on the train", "on the plane", "on a ship", but "in the car".
Some try to explain this with "enough room to stand up = on; have to sit down = in" or "whether board makes sense" ( http://ell.stackexchange.com/questions/19761/why-do-you-get-in-a-car-but-get-on-a-train ), and for some modes of transport the preposition isn't quite fixed (e.g. "on/in a helicopter?").
So in English "I forgot my bag on the bus" would usually mean the bag is inside the bus, but could mean it's on the roof; but "I forgot my bag on the car" can only mean that the bag is on the roof.
Calling these suffixes "cases" is unnecessarily scary. Cases are for Indo-European languages, and are far more complicate than these, but Hungarian belongs to another family.
In Hungarian, PREpositions are attached after the words, instead of before, therefore they could be called POST-POSITIONS (or "postposition suffixes"). That is all.
We generally tend to call them suffixes (-ban, -nál, -val, -n, and so on) and postpositions (előtt, felé, között, szerint, and so on).
About the explanation of round vowels:
When I pronounce U and Ü (or O and Ö) in front of a mirror, my lips are identical: it is the tongue inside that changes position.
I gave the correct answer, but it showed me, that I am wrong. I got the same translation from duolingo like mine. I reported it already and hope, it will not happen with everybody.
It happened to me during another lesson. I found out it was an internet connection problem.
PS: no offense meant, but since we are talking about languages, I think you should refresh the use of commas in English. Specifically: you should not put a comma at the beginning of an object subordinate sentence. You did it twice in two lines. No offense!
"Correct a fool, and he will hate you; correct a wise man, and he will thank you [for improving him]" -- Book of Proverbs, page 9
Thank you :-) for your help! Now I know, that the German comma rules don't work in English (french, Italian and Hungarian) It was a hard decision for me to choose whether I want to be a fool or a wise woman :-)) (without comma!). The problem is, that I never understood grammar, numbers and music notes and have to learn languages and instruments with learning by doing (including mistakes). I do not even know what an object subordinate is. On the other hand no problems with pronounciation in different languages. Maybe I have to remain a kind of fool :-)) , due to my lack of understanding and/or remembering grammar rules.
Nobody is perfect. I have lots of limits, too. For example, I tend to use unnecessarily complicated words (simply put, I meant to tell you: no comma before the word "that"). I hope someday you will kindly correct a mistake I will inevitably do. Karma comes back. Cheers!
:-) OMG This should be so easy to remember, thank you! In future I will!