Learn Swedish: Vems är stöket på vandrarhemmet? - Whose is the mess at the hostel?

When vems (whose) is directly attached to a noun, that noun must be indefinite, like in vems stök är det? (whose mess is it?). But in this sentence, vems stands alone and is separated from the noun by the verb är. When this happens, you use the definite form because you are pointing to a specific mess: Whose is the mess?

What is the breakdown of the word vandrarhemmet?

It is a compound of vandrare (wanderer or hiker) and hem (home), which translates to hostel. Because hem is an ett-word, the whole compound is an ett-word, giving it the definite ending -et. Notice it gets an extra m at the end (-met) to keep the vowel sound short.

Why use the preposition på here instead of i (in) or vid (at)?

For temporary accommodations and many public buildings like a hotel (hotell), a restaurant (restaurang), or a hostel (vandrarhem), Swedish conventionally uses the preposition på to mean at or in.

Is stök just any kind of dirt or mess?