"We work when they sleep."
Translation:Vi arbejder når de sover.
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.
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
I work when they sleep. I work while they sleep.
When means a point in time, while means a period in time.
If it's a period I would suggest using while, like in this example I work while they sleep is better.