Tricky one: some examples
"Ich kann im Bett nicht schlafen." Means, I cannot sleep in a bed, I should better sleep on a floor mattress for instance.
Well, even if "im" is basically the same like "in dem!" , if the German speaker chooses "in dem" it equals more or less: "in diesem" at this means: "in this particular bed" and changes the meaning dramatically to: "I cannot sleep in this bed."
Another example where a context would have helped.
I would go here with the translation suggested by the green OWL.
Using diesem would put emphasis on THIS. As in, I can not sleep in this bed, I should sleep on another bed, etc. This way just seems like a more descriptive way of saying "I can't sleep." It's like "I can't go." and "I can't go to the party." Same thing, just more descriptive.
I've come across other excercises like this with a preposition that takes the dative case, and the explanation given for why the article is in the dative case reads:
"When a word is the indirect receiver of an action it gets a special article. For example The girl gives the child the book is Das Mädchen gibt dem Kind das Buch."
This isn't the rule that applies here, is it? This might be confusing for learners who aren't aware of the different instances where the dative case of articles should be used.
I think explanations like this are helpful, but they have to be correct. And preferably provide a more thorough explanation, e.g. with "article in the dative case" rather than the mystifying "special article". Or at least a link to a page that explains the grammatical rules thoroughly.
In is a two-way preposition. It takes the dative when there is no movement (or there is movement in the same place) and takes the accusative when there is movement. Here it is explained now: https://www.duolingo.com/skill/de/Accusative-Prepositions
You may be looking for "Ich muss in den Bett nicht schlafen", which translates to the next best way to say what youre describing in English, "I dont have to sleep in the bed." Although i am curious if you can do like in English, and simply emphasize the "nicht" in the original sentence.
No, it is not! ;-)
Because nobody can see that you have a capital S in schlafen.
When you write it like this: "Ich kann nicht in dem Bett schlafen" your are perfectly fine.
Alternatively: "Ich kann in dem Bett nicht schlafen"
If you want to stress/emphasize a particular Bett it sounds like:
"Ich kann in diesem Bett nicht schlafen"
The thought is absolutely correct. I have never heard it expressed this way, but I can well imagine it being used (especially for emphasizing).
Some additional information: You say "auf einer/der Couch schlafen" (and not "in") if you are sleeping on a/the couch, although the friend you are staying with gave you something to cover up.