This may be a daft question, but how do I know whether the sandwiches are singular or plural?
In this sentence, you don't. You have to get that information from context.
Takk for det!
Sorry, people may have already answered this kind of question in other topics but but what's exactly the difference between "Guttens smørbrød" and "Guttens smørbrødet" ?
"Guttens smørbrødet" does not work. It would be equivalent to saying "The boy's the sandwich".
It may work if the boy was actually a sandwich hahaha
I dont understand why the boy's sandwich is wrong?
It shouldn't be. Maybe you made a typo?
I also wrote ' The boy's sandwich', and got correction: The boys sandwich', which is not correct. Problem within Duolingo, I think.