Mig langaði í ananas eftir kvöldmat, en hana langaði frekar í hindber.

Because langa is used here as an impersonal-style experiencer verb.

With this pattern:

• the person who feels the desire is in the accusative
• the verb stays in 3rd person singular
• the thing wanted comes after í

So:

• ég → nominative I
• mig → accusative me

That is why Icelandic says Mig langaði ..., literally something closer to Me wanted ... than I wanted ....

The same pattern appears in the second clause:

• hana langaði = she wanted / she felt like

Because langa takes the person in the accusative, not the dative.

For hún:

• nominative: hún
• accusative: hana
• dative: henni

So after langa, you use hana.

This is a very common point of confusion because some other Icelandic feeling-verbs take dative instead. For example:

• Henni líkar... = She likes...

But:

• Hana langar... = She wants / feels like...

So the case depends on the verb.

Langaði is the past tense form of langa in this construction.

A useful contrast is:

• Mig langar í ananas. = I want / feel like pineapple.
• Mig langaði í ananas. = I wanted / felt like pineapple.

In this pattern, the verb usually appears in 3rd person singular, even if the person wanting something is mig, þig, okkur, hana, and so on. That is because the verb is not agreeing with a normal nominative subject.

So you get:

• Mig langaði
• Hana langaði
• Okkur langaði

all with the same verb form.

Because Icelandic normally uses the pattern langa í + noun when talking about wanting a thing, especially food or drink.

So:

• Mig langaði í ananas
• Hana langaði í hindber

Here í is part of the normal expression. It does not literally mean into in the way it often does in other contexts.

Think of it as a chunk you learn together:

• langa í eitthvað = to want / feel like something

The noun after í is in the accusative, though in these examples the form may look the same as the dictionary form.

Use:

• langar í + noun for a thing
• langar að + infinitive for an action

Examples:

• Mig langar í kaffi. = I want coffee.
• Mig langar í ananas. = I want pineapple.
• Mig langar að borða ananas. = I want to eat pineapple.
• Mig langar að fara heim. = I want to go home.

So in your sentence, í ananas and í hindber are used because the speaker wants a food item, not an action.

Frekar means rather, preferably, or instead depending on context.

In this sentence, it shows a preference between two possibilities:

• Mig langaði í ananas
• en hana langaði frekar í hindber

So the idea is:

• I wanted pineapple,
• but she would rather have raspberries.

Its position is very natural here: frekar comes before the í + noun phrase it modifies.

Here eftir means after, and in this time expression it takes the dative.

So eftir kvöldmat means after dinner.

A helpful detail: the noun kvöldmatur has a dative singular form kvöldmat, and that happens to look the same as its accusative singular form. So even though the case is dative here, you do not see a different ending.

That is very common in Icelandic: sometimes the case is important grammatically, even when the form does not visibly change.

Because Icelandic often uses bare nouns in this kind of sentence, especially with food.

So:

• í ananas
• í hindber

can sound perfectly natural without an article.

English often wants something like some pineapple or raspberries, but Icelandic does not always need an extra word there. The noun by itself can express the general idea of what someone felt like eating.

If the speaker meant a very specific, already-known item, Icelandic could make that more definite, but in ordinary sentences like this, the bare noun is normal.

In this sentence, the form hindber does not clearly tell you by itself.

What matters for a learner is:

• the word used is hindber
• in this structure, the form stays hindber
• context or translation tells you whether English should say raspberry or raspberries

With fruit and food, English often prefers a plural or mass-noun translation, while Icelandic may simply use the noun form without making that distinction obvious in the sentence.

So if your translation says raspberries, that is perfectly natural.

You might hear or understand something like that, but mig langaði í ananas is the more natural pattern for feeling like a food.

A rough difference is:

• Mig langaði í ananas = I wanted / felt like pineapple
• Ég vildi ananas = I wanted pineapple

The version with langa í is especially common for cravings, preferences, and food/drink choices. It sounds very idiomatic.

So for this kind of sentence, learning mig langaði í... as a set pattern is a very good idea.

It is a very natural Icelandic word order.

The second clause is:

• en hana langaði frekar í hindber

Breakdown:

• en = but
• hana = her
• langaði = wanted / felt like
• frekar = rather
• í hindber = raspberries

Placing frekar before í hindber makes it clear that the preference is about what she wanted. In English we often say she rather wanted... less often than she would rather have..., but in Icelandic this placement is completely normal.

So nothing strange is happening there; it is just standard Icelandic sentence structure.