"The athlete does not run in onto the grass, but sits down onto the ground."
Translation:A sportoló nem fut be a fűre, hanem leül a földre.
I think it's because you are negating (nem) the futing, not the be.
If you wanted to contrast nem befut, hanem lefut or something like that, I think you would have nem before the preverb.
I'm not at all sure, so this is speculative, but comparing this sentence with:
"A gyerekek nem lesétálnak az utcára, hanem leugranak"
it seems that perhaps here (unlike in the linked sentence) we don't have a direct contrasting of two verbs, just a juxtaposition of two situations, so here the focus is on "nem" (like in most sentences with a negation) and hence the prefix is separated from the verb.
In any case, this is a fascinating (if, to me, slightly elusive) topic.
You don't usually "sit down onto the ground" in English "sit down on the ground" is how we would normally say it.
That's what I've been saying all along! Thank you, Coralie! These guys are speaking some other language I'm not familiar with...
WTF is "run in onto" mean? What makes you file up all these words nobody ever uses? Is it just to confuse us all?
We think that the athlete is running on the pavement and then goes into an area covered with grass, where he keeps running onto the grass. BUT no, he is not! he just sits down onto the ground.