"Who sleeps on that bed?"
Translation:Wer schläft in diesem Bett?
Actually, Duolingo considers this sentence wrong, even with diesem. "auf" is apparently incorrect, and one should use "in"
duolingo's tendency to interchange definite articles with demonstratives seemingly at random is causing me problems.
Sometimes "der" means "the", except when it means "that". I understand the uses, but it's a sometimes a guess whether one is going to be counted as correct or not.
Your translation is also correct. Send them a reply. Infact your translation is even better. on the bed - auf dem Bett, in the bed - in dem Bett
"an diesem Bett" - am = an + dem. You don't need "dem" if you've got "diesem". I don't whether they would accept "an diesem" (don't trust me on which preposition to use.
They accepted "auf", and said "in" was another correct translation; I don't know about "an".
"Das" means that as a pronoun, not as an adjective. So you can say "Welche ist dein? (Which is yours?)" "Das ist mein. (That is mine.)" But not "Das Bett ist mein (which translates to THE bed is mine)". You must say "Dieses Bett ist mein."
dass is the subordinating conjunction "that"
Ich weiß, dass er müde ist. = I know that he is tired.