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.

Because í can take either the accusative or the dative, depending on meaning:

• accusative: movement toward something, change of state, or figurative direction/purpose
• dative: location, being inside something

Here the idea is not in the dessert, but rather for dessert / as dessert, so Icelandic uses the accusative:

• í eftirréttinn

If you said í eftirréttinum, it would sound more like in the dessert or in the dessert course, which is a different idea.

The base dictionary form is eftirréttur (dessert).

This noun changes form:

• eftirréttur = nominative singular
• eftirrétt = accusative singular, indefinite
• eftirréttinn = accusative singular, definite

The ending -inn is the suffixed definite article, meaning the.

So:

• eftirrétt = dessert
• eftirréttinn = the dessert

Kaupum is the 1st person plural present tense of the verb kaupa (to buy).

So:

• ég kaupi = I buy
• við kaupum = we buy

That -um ending is a very common marker for we in the present tense.

Because they are being used indefinitely here: the sentence is talking about buying some raspberries and pineapple, not specific already-known ones.

In Icelandic, the definite article is usually attached to the end of the noun. So if you meant specific fruit, you could have forms like:

• hindberin = the raspberries
• ananasinn = the pineapple

But here the bare forms hindber and ananas are the natural choice.

They are the direct objects of kaupum, so they are in the accusative.

That said, in Icelandic some nouns look the same in more than one case, so you do not always see a visible ending change. In this sentence, the object function tells you they are accusative.

In context, it is usually understood as plural: raspberries.

A useful thing to know is that some Icelandic neuter nouns have the same form in singular and plural in certain cases, so the form alone does not always tell you the number clearly. Context does the work.

So here hindber is best understood as raspberries, even though the written form itself can be ambiguous.