That is just how it is. If you're confused, it's because you're looking for a system and not finding one. But prepositions are like that. There's not really any satisfactory explanation, because nobody sat down and devised the language. It just happens that Swedes find many good uses for på. The only vaguely helpful thing I can say is to just think of it as the English word on, and accept that the Swedes prefer to say "on the school" rather than "at school".
In this case Duo accepts/suggests the translation "during workdays" which frankly comes across as a slightly unnatural to me. I'd say that "on workdays" sounds much more natural in English, and then it's actually the same preposition as we'd use in English (på = on). I'm sure that's what I wrote and was also accepted.
"They work on workdays." is also accepted as correct by Duolingo. They may have wanted us to know that even if we used "during" in English that it would still be "på" in Swedish.
It is accepted, but your error report says "the work on workdays". You need "they", not "the".
Yes. As a native English speaker I find the course's default translation "workdays" quite odd. Chambers Dictionary says workday can mean weekday, but it's very unusual. I wasn't even positive it was a real English word until I looked it up.
A weekday is any day of the week except for Saturday or Sunday. A workday is any day that you regularly work, for some people in Norway that includes Saturday and this does not include weekdays which have been designated as holidays.
Why does it sound like dem(dom) instead or de? Is 'de' also pronounced like dem(dom)?
Try reporting it: http://www.merriam-webster.com/dictionary/businessday
I might be confused, but I think I saw this being used as "everyday", was I mistaken?
You're probably thinking of vardaglig. The concepts are pretty closely related.
Them work would not be grammatical in English; It could only be They work. The objective case is them and this sentence isn't using an object.
Prepositions have a habit of confusing. I would not use in. I would use on with days of the week.
As a non native english speaker I'm having sooo much trouble understanding the particles. I mean, I know the theory and why and how. It's just not natural for me to use them at all. Lately I got it in Swedish, but now I have more troubles in English. And in tasks like this I forget to write them. Does anyone have any idea how to get on the bottom of this for those whose native language has cases(forms) and to be more specific, slavic origins. It's making me crazy.