Hún vill frekar hafragraut en morgunkorn þegar veðrið er kalt.

Frekar ... en ... is the standard Icelandic pattern for rather ... than ....

So:

• vill frekar hafragraut en morgunkorn = prefers oatmeal rather than cereal
• more literally: wants oatmeal rather than cereal

In Icelandic, vilja often covers both want and, in some contexts, prefer, especially when combined with frekar.

Icelandic often expresses preference with vilja frekar rather than a single verb exactly matching English prefer.

So instead of saying something structurally identical to English she prefers X to Y, Icelandic commonly says:

• Hún vill frekar X en Y

This is a very natural and common way to express preference.

Hún means she, and vill is the 3rd person singular present form of vilja (to want).

A few present-tense forms of vilja are:

• ég vil = I want
• þú vilt = you want
• hann/hún/það vill = he/she/it wants

So hún vill simply agrees with hún.

They are in the accusative case, because vilja normally takes a direct object in the accusative.

• dictionary form: hafragrautur
• here: hafragraut (accusative singular)

Morgunkorn is neuter, and its form is the same in nominative and accusative singular, so it does not visibly change here.

Because it is a masculine noun in the accusative singular.

The dictionary form is:

• hafragrautur = oatmeal / porridge

But as the direct object after vill, it becomes:

• hafragraut

This kind of change is very common with masculine nouns in Icelandic.

Because many neuter nouns have the same form in the nominative and accusative singular.

So:

• morgunkorn = cereal
• nominative singular: morgunkorn
• accusative singular: morgunkorn

Even though it is also a direct object, the form happens to stay the same.

Veðrið means the weather, while veður means just weather in a more indefinite sense.

The ending -ið is the suffixed definite article for this neuter noun:

• veður = weather
• veðrið = the weather

In this sentence, Icelandic uses the definite form naturally: when the weather is cold.

The adjective agrees with the noun it describes.

Here the subject is:

• veðrið = the weather

Veður is a neuter singular noun, so the adjective must also be neuter singular:

• masculine: kaldur
• feminine: köld
• neuter: kalt

That is why the sentence has veðrið er kalt.

Because after þegar (when), Icelandic uses a subordinate clause, and the word order is usually more straightforward than in main clauses:

• þegar veðrið er kalt = when the weather is cold

In a main clause, Icelandic often has verb-second word order, but subordinate clauses introduced by words like þegar often keep the subject before the verb if there is no special reason to move anything.

Yes. Þegar can mean:

• when as a conjunction
• already as an adverb

In this sentence, it clearly means when, because it introduces a clause:

• þegar veðrið er kalt = when the weather is cold

If it meant already, it would be used differently in the sentence.

Icelandic often leaves nouns indefinite where English might or might not use an article, especially with foods in a general sense.

So:

• hafragraut = oatmeal / porridge
• morgunkorn = cereal

The sentence is talking about these foods generally, not about one specific bowl of oatmeal or one specific cereal. That is why no definite ending or separate article is used.

Usually yes in this kind of sentence, but it is often closer to porridge or oat porridge in meaning.

Literally:

• hafra- = oat
• grautur = porridge / mush

So hafragrautur is the cooked oat dish, not dry oats.

Yes, but the original word order is very natural.

For example, you could move the time clause to the front:

• Þegar veðrið er kalt, vill hún frekar hafragraut en morgunkorn.

Because Icelandic is a verb-second language in main clauses, once the þegar clause comes first, the finite verb vill comes before the subject hún in the main clause.

So:

• main clause first: Hún vill ... þegar ...
• subordinate clause first: Þegar ..., vill hún ...

Both are correct.

A learner will often wonder about ð and þ.

In this sentence:

• Hún: ú is like a long oo
• veðrið: ð is a voiced sound somewhat like th in this, though in Icelandic it can be weaker depending on position
• þegar: þ is like th in thing

So:

• þ = unvoiced th
• ð = voiced th

That distinction is very important in Icelandic spelling and pronunciation.