Questions & Answers about Í eftirréttinn kaupum við hindber og ananas.
Because Icelandic main clauses usually follow the verb-second (V2) rule. That means the finite verb comes in the second slot.
Here, the first slot is already taken by Í eftirréttinn, so the verb kaupum has to come next:
Í eftirréttinn | kaupum | við ...
After that comes the subject við.
So this is normal Icelandic word order. If you started with the subject instead, you could also say:
Við kaupum hindber og ananas í eftirréttinn.
That version is also correct, but it gives less emphasis to for dessert.
It is a prepositional phrase:
- í = in, into, for depending on context
- eftirréttinn = the dessert in the accusative singular
In this sentence, the phrase is used idiomatically to mean for dessert or as dessert. So it tells you the purpose/use of the fruit, not just location.