"We work when they sleep."
Translation:Vi arbejder når de sover.
The difference between mens and når in this case, is the same as between while and when in English. It's a small subtle difference, and both examples sound right in Danish.
It should be accepted. Duolingo's problem. I am 100% fluent in Danish (it's my mother tongue) and when I answer these "translation" questions, I always get it wrong because the questions aren't open to several translations and are directed towards people who aren't fluent like me.
Yes, it is only used as a question.
Note that Hvem (who), Hvad (what), Hvor (where), Hvorfor (why), Hvordan (how) all are used in both contexts: as questions and in subordinate clauses...
Actually, 'hvornår' can also be used in a subordinate clause. But only when 'when' means 'at what time'. When 'when' means "at such times at", "at the time of [...]" or "since/given the fact that". In these cases, you'll use 'når'
Han ved ikke hvornår hun kommer - He doesn't know when/at what time she will come
Han arbejder når de sover - he works when/at the times they sleep
Hvordan kan der være en gud, når der sker så meget forfærdligt - how can there be a God when/given the fact that so many horrible things are happening
My answer was correct, the same as others prior to this, however, my internet connection was broken automatically, so Duolingo repeatedly registered my answers as incorrect.
There were a couple of typos, I must admit because there seems to be a bit of dyslexia (which I do not own). Anyway, I tend to mix letters in spelling though I know what I mean to convey.