Translation:We are not sitting down on the table, but on the chairs.
yes, I see some contradiction between using a verb which seems to indicate a continuous state "ül" (by opposition to a change of state: "felül"or "leül", and a suffix which (to my understanding) seems to indicate the result of an action/motion: "ra/re" (onto) by opposition to a position: "en/on":. I have noticed this in several exercises here. Maybe I misunderstand something. Could it be that the suffix replaces the preverb?, but then why "sitting down onto" as seen here?
This is my observation rather than a rule I've learned from a book, but it seems like you'll usually see a preverb omitted if it comes immediately after a suffix that says that same thing - mostly just because the repetition would sound awkard. So you get
Bemegyek az épületbe
Az épületbe megyek
but not *Az épületbe bemegyek.
So that's what's going on in the Hungarian half of the item, at least to my best understanding. The verb here pretty much naturally ends up after Nem az asztalra, but when it's in that immediate proximity to -ra there is no need to put a preverbal marker on it as well.
On the other hand, I would say that Leülünk nem az aszaltra, hanem a székre is also possible (most likely in speech where the order of the words wasn't planned out like you would in writing) and there it's natural to have a preverb.
The question of which prepositions to include in the English translation is a whole different question.
Replying to jsiehler's observations here: very good! I have some further comments though.
The "be-" preverb still carries the sense of completeness. And if you omit it for whatever reason, that sense is gone. So
"Bemegyek az épületbe" - I am entering the building.
"Az épületbe megyek" - I am going to enter the building. Probably. But the emphasis here is on the building and not so much on the fact that I am going to enter it. So this one answers to "Where are you going?" as opposed to "What are you doing?" above.
If you say
"Az épületbe bemegyek", it is a special emphasis. It could be an answer to "What are you doing with this building?" - "This building I am going to enter".
Or you could look at several buildings and make decisions whether to enter them. You look at the first one... "I am not entering this one"... you look at the second one "Nah, I am not entering this one, either"... But the third one catches your eye... "I am going to enter this one!" :
"Ebbe az épületbe bemegyek!"
On "Leülünk nem az asztalra, hanem a székre". It is incorrect to say like that. In speech, one could say it with a pause though:
"Leülünk. Nem az asztalra, hanem a székre."
"Leülünk, nem az asztalra, hanem a székre."
"Leülünk, de nem az asztalra, hanem a székre."
"Nem az asztalra ülünk (le), hanem a székre."
If we are translating surely we need to use a sentence that is grammatically correct! We don't sit down onto a chair or a table for that matter. We sit down at a table unless our bums are physically on the table and we sit down on a chair. I do understand the use of ra and re but I physically cringe at having to write such a distorted form of English!
I have observed the same as Jsiehler in many many cases here: "be" not being repeated as a preverb directly after a "be" suffixe. However, when practicing, I came across the following sentence: Two dogs run into the store. Proud of my new knowledge, I translated: két kutya az üzletbe fut. To my dismay, this was rejected and the "correct" solution was: két kutya az üzletbe befut. Back to square one, I guess.
Well, don't discount Duo being the wrong one just yet! :) "Két kutya az üzletbe befut" sounds quite weird to me. I would rather go with your version
"Két kutya az üzletbe fut" - but this one is missing the completeness factor, that is, they are running toward the door with the full intent to go inside. But that is all we know.
"Két kutya befut az üzletbe" - on the other hand, means that the dogs are entering the store right now.
You are welcome. There was some extended discussion on this topic earlier in the course, maybe you missed out on them. Just one example: https://www.duolingo.com/comment/16775767
If you want a general feel for this phenomenon, look up telicity (apparently there is a scientific term for it). It appears in a few languages. Also, you can read up on perfective prefixes.
Please stop digging another hole : this is another dreadful translation into english regarding preps. I appreciate the course and the hard work involved but some of these translations..........istenem!