The real issue here (inasmuch as we're trying to learn German, not English) is what the German sentence means.
And what strikes me here is that it appears that German has an easy and natural way of expressing a distinction that - at best - is not so easily and naturally expressed in English: we don't have the cases and perhaps not the words for it.
Which does not mean, I hasten to add, that Bobby should get away with writing auf das Sofa.
I'm English too :)
I know onto indicates movement but so does the use of the accusative case in this German sentence, which is precisely why I used it as a literal translation. The ink moves from the pen onto the sofa. As I said, I agree that it's not conventional English and not the way that you would usually say this in English. You can, however, resort to it to make the distinction clear.
-- "Bobby's been writing on the sofa again."
-- "So? He's probably more comfortable there than at his desk."
-- "No, I mean he's written onto the sofa."
It does work. Here, the German sentence wants us to understand that it's "auf das" not "auf dem". Using the slightly clumsier and more awkward English construction lets you demonstrate that in my opinion. As I said, I guess it depends on what you think Duolingo is asking you to do.
It does. It's just not terribly conventional because we usually make this distinction from context or with an extra qualifier (e.g., "He wrote on the sofa itself").
If you want to explain the distinction between "auf dem Sofa" and "auf das Sofa" to a native English speaker, however, it works pretty well.
The question is whether Duolingo wants to test whether the user understood the German or whether they can produce a good English translation. In the case of ambiguous translations like this, the two aren't necessarily the same.
My answer, I'd argue, demonstrates that I understood the case being used but it doesn't mean I would necessarily say it that way in English. I've frequently found that to the best approach to use when answering questions on here.
And to avoid the question in the other sense: "He was on the sofa writing." Not the same tense, but still in the past and clear that he wasn't writing on the sofa... "He wrote on the sofa," can clearly go both ways with context necessary to make the distinction. Idn't dis fun?
A way of checking whether it is dative or accusative: If the preposition can be used with just the subject, it is dative, if it can't it's accusative. For example: "The sensible adult has written their report on the sofa"– you could say "the sensible adult on the sofa". But… "The naughty child has scribbled graffiti onto the sofa" – you can't say "the naughty child onto the sofa", it doesn't make sense.
This is only a rule of thumb so naturally there are exceptions. (Für is one exception – you could say "the letter for the Prime Minister" which would be der Brief für den Premierminister)
It's not just movement in itself but movement towards something, that calls for the accusative case. So, graffiti being written on the sofa would be 'aufs Sofa' but writing a letter, say, whilst on the sofa would be 'auf dem Sofa' as the sofa is not the recipient of the movement of the writing implement.
It means that the ink/graphite/whatever crayons are made from etc. from his pen/quill/pencil/crayon etc. ended up on the sofa. He may have been sitting on the sofa or not, and he may have ruined it, or he may have improved it; all we know is that the ink etc. from his pen etc. ended up on the sofa in the shape of letters (or some other writing system), and this was the intention of an unidentified male.
Because the 'movement' the sentence is capturing is the transferral of ink to the sofa, specifically. If it was "auf dem Sofa" the 'action' the sentence would be capturing is the action of sitting in one spot, on the sofa, whilst writing (but the writing is not the point of the sentence, the thing the speaker is pointing out is that the subject immobile, sitting, and happens to be writing). But since it's "aufs (split into 'auf das') sofa," we know that the information the speaker is trying to portray is in the Akkusativ case, which means whoever is doing the writing is indeed NOT just sitting there, immobily writing, he is, in fact, writing auf das Sofa (his writing utensil is leaving marks on the sofa.)