Translation:Why are the things lying on the floor?
This is consistently true in Swedish from what I can tell, but not always the case in idiomatic English in some dialects. In some parts of the US for example 'ground' can refer to the floor of a building as well in some cases (though the reverse is almost never true). For example 'on the ground' in Midwestern American English says nothing about whether it's indoors or not in most cases.
That would be or Varför ligger (or finns) det saker på golvet? In this sentence, we're talking about some previously not known items, but the sentence with sakerna is talking about some specific saker which are previously known to the speaker and listener. One sentence is about the floor, the other about the things, so to speak.
I agree with cheezwizard93 on this one, though there is nothing wrong with the suggested construction other than it doesn´t feel natural. Just ask my husband or millions of children lol). I do appreciate understanding the Swedish perspective, though, so thanks for for that.
If both you and I know what the things are, the most natural would be ´Why are those things on the floor?´or ´Why are there things on the floor?´Third choice for frequency or naturalness in English would be the construction with ´the things´.
It isn't a special Swedish perspective, but you're probably right that other sentences would be more common in real life. It's just that Duo's idea is to teach grammar patterns rather than "useful sentences".
Your sentences would be Varför ligger de där sakerna på golvet? and Varför ligger det saker på golvet?, respectively.