why not "This" ?
this in Swedish is det här or detta.
det in Swedish usually means it in English, but it can sometimes also mean that, so those are the accepted translations in the course.
How do you know when to put "det" or "de"?
" Det " is when it's one, " De " is when it's many.
I think this 'det' is the same as in 'det regnar'.
Can 'klänning' also mean dress as in 'they way you dress' or 'national dress,' etc.?
I wrote klenning/accepted. I feel that this Swedish course accepts way to many errors as typos. :-(
It's not the Swedish course, it's Duolingo itself :-( . Duolingo has difficulty with the difference between a/ä/å and o/ö, cannot distinguish them properly. The same thing happens in the German course. I hope Duo marked your mistake is as a typo.