I know the Danish translation team probably have limited control over the software, but as a native Danish speaker, I could not hear "sandwiches" at all, and consequently got the exercise wrong, because I just heard 'sandwies', like Reisam said.
It's an error with the tree, which we will try to correct soon. The correct plurals of sandwich are: sandwich and sandwicher. However unofficially sandwiches are more used, which is why the error entered the course. Due to regional words :)
Because it begins with Spiser and not with Du. Danish is not my native language, but this is the same in dutch. When Du is at the beginning, you say: you eat sandwiches. and when Spiser is at the beginning, you say: do you eat sandwiches? I hope you understand now :-)