Hvad er forskellen mellem "sid ned" og "sæt dig ned"?
There is no difference, and both are used in common speech. The latter literally means "set/put yourself down".
The tooltips offer "sit" as a translation for "sid ned" but it is not accepted as the answer.
...is it just me, or is her accent uniquely difficult to understand in this one?
Sæt dig her ved mig.
It's really annoying when one question has 'sid ned' as sit down and the next has 'saet ned' but you are marked wrong if you don't happen to guess which one DuoLingo wants. Why not accept both?
I don't think "sæt ned" is correct - you are missing a subject.
"Sid ned" = literally "sit down".
"Sæt dig ned" = "set yourself down", used to mean the same thing as "sit down". But the second word (dig) is essential to the meaning.