Jag köper havregryn igen, för barnen vill ha det till frukost.

In Swedish, the present tense often covers both I buy and I am buying. So Jag köper havregryn igen can mean either I buy oats again or, more naturally in context, I’m buying oats again.

Swedish does not usually need a separate continuous form like English am buying. If you really want to stress that something is happening right now, you can add words like nu (now), but it is often unnecessary.

Igen means again.

So Jag köper havregryn igen means that this is happening one more time:

• I’m buying oats again
• I have to buy oats again

It often gives a slight feeling of repetition, sometimes because the item has run out or because the same situation keeps happening.

Havregryn means oats or rolled oats, the kind often used for porridge.

A few useful points:

• havre = oats
• gryn = grain / groats

Together, havregryn is the normal word for oats as food. In this sentence, it is treated as a mass noun, similar to how English uses oatmeal or oats depending on context.

Barnen means the children.

The base word is:

• ett barn = a child
• barn = children
• barnen = the children

The ending -en makes the plural definite. Since the sentence is talking about specific children already known in the situation, Swedish uses barnen.

Here för means because.

So:

• Jag köper havregryn igen, för barnen vill ha det till frukost.
• I’m buying oats again, because the children want it for breakfast.

In everyday Swedish, för is common in speech and informal writing for because. You may also see därför att, which is a fuller version:

• ..., därför att barnen vill ha det till frukost.

Because this is a normal subordinate explanation after för, and it keeps standard statement word order:

• subject: barnen
• verb: vill
• infinitive: ha
• object: det

So:

• barnen vill ha det = the children want to have it

If you start a main clause with another element, Swedish often uses inversion, but that is not what is happening here.

Vill ha literally means want to have, but in many contexts it is the natural way to say want something.

So:

• barnen vill ha det = the children want it

This is broader and more idiomatic than vill äta det.
Vill äta det would specifically mean want to eat it, while vill ha det till frukost means they want it as their breakfast food.

Det means it, and it refers back to havregryn.

Swedish often uses det to refer to nouns that are:

• neuter singular, or
• mass nouns / uncountable things

Here havregryn is being treated as a thing/substance, so det is natural:

• havregryn → det

Repeating the noun would sound less natural:

• ... barnen vill ha havregryn till frukost is possible, but det is smoother because the noun was just mentioned.

It is an ett word:

• ett havregryn = an oat / an oat flake

That helps explain why the pronoun is det:

• ett noun → often referred to as det

Even though English speakers may think of oats as plural, Swedish havregryn is often used more like a substance or category of food.

Till frukost means for breakfast.

Swedish often uses till when talking about food eaten for a meal:

• till frukost = for breakfast
• till lunch = for lunch
• till middag = for dinner

So barnen vill ha det till frukost means they want it as the food they will eat at breakfast.

Usually, no. The natural expression here is till frukost.

• till frukost = for breakfast / at breakfast
• på frukosten sounds unnatural in this meaning
• för frukost is also not the normal choice here

So if you are talking about what someone wants to eat as breakfast, till frukost is the standard phrasing.

Because Swedish modal verbs usually take the infinitive without att.

Vilja is one of those verbs:

• Jag vill ha det = I want to have it / I want it
• not Jag vill att ha det

Other common modal verbs work the same way:

• kan = can
• ska = will / am going to
• måste = must
• vill = want to

So barnen vill ha det is exactly what you should expect.

Yes, it can.

Because of igen (again), the sentence can simply describe repetition, but depending on tone it can also suggest:

• Here we go again
• I have to buy oats again
• mild annoyance or resignation

The grammar itself is neutral, but intonation and context can give it that extra feeling.

Yes. A few natural translations are:

• I’m buying oats again, because the children want them for breakfast.
• I’m buying oatmeal again, because the children want it for breakfast.
• I have to buy oats again, because the children want them for breakfast.

The best English version depends on context and on what kind of havregryn is meant. Swedish itself is straightforward, but English may choose oats, oatmeal, or sometimes porridge oats depending on the situation.