Breakdown of Vems är den där blå necessären?
vara
to be
blå
blue
den där
that (en)
vems
whose (question word)
Questions & Answers about Vems är den där blå necessären?
Why does the verb är come directly after vems here?
Vems (whose) acts as a standalone question word in this sentence. Because Swedish requires the verb in the second position (the V2 rule), the verb är must immediately follow it. You could also group the noun with it and say Vems blå necessär är det där? (Whose blue toiletry bag is that?), where the whole phrase acts as a single block in the first position.
Why do we use the definite form necessären after den där?
This is a classic example of Swedish double definiteness. When you use demonstrative pointers like den där (that) or den här (this), the grammar requires you to also put the noun itself into its definite form by adding the -en suffix.
Since it is in the definite form, why isn't blå written with an -a ending as blåa?
Generally, adjectives get an -a ending when they are in the definite form. However, for short adjectives ending in a stressed vowel like blå (blue) or grå (gray), the -a is completely optional. You will frequently see and hear both den blå necessären and den blåa necessären.