Na śniadanie jem owsiankę z miodem, a moja siostra wybiera płatki z jogurtem.

Na śniadanie is the normal Polish way to say for breakfast when talking about what someone eats.

• na
  • accusative is often used for meals:
    • na śniadanie = for breakfast
    • na obiad = for lunch/dinner
    • na kolację = for supper/dinner

So Na śniadanie jem owsiankę means For breakfast, I eat porridge or more naturally I eat porridge for breakfast.

W śniadaniu would mean something more like in the breakfast, which is not what you want here.

Because jem takes a direct object, and the direct object is usually in the accusative case.

• dictionary form: owsianka
• accusative singular: owsiankę

So:

• jem owsiankę = I eat porridge

This is very common with feminine nouns ending in -a:

• kanapka → jem kanapkę
• sałatka → jem sałatkę
• zupa → jem zupę

After the preposition z meaning with, Polish usually uses the instrumental case.

That is why:

• miód → z miodem
• jogurt → z jogurtem

So:

• owsianka z miodem = porridge with honey
• płatki z jogurtem = cereal/flakes with yogurt

This pattern is very common:

• kawa z mlekiem = coffee with milk
• chleb z masłem = bread with butter
• makaron z serem = pasta with cheese

In this sentence, płatki most naturally means breakfast cereal or flakes.

On its own, płatki can refer to different kinds of flakes, depending on context:

• płatki śniadaniowe = breakfast cereal
• płatki owsiane = oat flakes
• płatki kukurydziane = cornflakes

Here, because the sentence is about breakfast, płatki z jogurtem is understood as cereal with yogurt.

Here a is a conjunction meaning something like:

• and
• while
• whereas

It often connects two contrasting or parallel ideas.

So:

• Na śniadanie jem owsiankę z miodem, a moja siostra wybiera płatki z jogurtem.

means something like:

• For breakfast I eat porridge with honey, and my sister chooses cereal with yogurt.
• or more literally, ..., whereas my sister chooses...

Polish a is often used where English would simply use and, but it usually suggests some contrast or comparison.

No, you do not always need moja.

• siostra = sister
• moja siostra = my sister

Polish often omits possessives when the meaning is clear from context, but including moja is very natural when introducing or emphasizing the person.

So both are possible:

• a moja siostra wybiera...
• a siostra wybiera...

Using moja makes it clearer and a bit more personal: my sister.

Wybierać is the infinitive, meaning to choose.

In the sentence, you need the conjugated form:

• (ona) wybiera = she chooses

Because the subject is moja siostra = my sister, the verb must be in 3rd person singular:

• ja wybieram = I choose
• ty wybierasz = you choose
• on/ona wybiera = he/she chooses

So:

• moja siostra wybiera płatki z jogurtem = my sister chooses cereal with yogurt

In Polish, subject pronouns are often omitted because the verb ending already shows who is doing the action.

• jem already means I eat
• wybiera already matches she chooses when the subject moja siostra is stated

So ja jem is possible, but usually only if you want emphasis or contrast:

• Ja jem owsiankę, a ona wybiera płatki. = I eat porridge, and she chooses cereal.

Without special emphasis, Polish usually prefers:

• Jem owsiankę...

Yes, Polish word order is fairly flexible, though some orders sound more natural than others.

The given sentence is very natural:

• Na śniadanie jem owsiankę z miodem, a moja siostra wybiera płatki z jogurtem.

You could also say:

• Jem na śniadanie owsiankę z miodem...
• Moja siostra wybiera płatki z jogurtem, a ja jem owsiankę z miodem.

But word order often changes focus or emphasis, not basic meaning.

Starting with Na śniadanie highlights the breakfast context first.

Not with the same meaning.

• wybiera = chooses / goes for
• lubi = likes

So:

• moja siostra wybiera płatki z jogurtem = my sister chooses cereal with yogurt
• moja siostra lubi płatki z jogurtem = my sister likes cereal with yogurt

The original sentence is about what she is choosing to eat for breakfast, not just what she likes in general.

Yes. Both are masculine nouns, and after z they go into the instrumental singular.

For many masculine nouns, instrumental singular ends in -em:

• miód → miodem
• jogurt → jogurtem
• ser → serem
• cukier → cukrem

So the pattern is normal. You just have to learn the noun and then apply the case ending.

Polish does not have articles like English a/an and the.

So:

• jem owsiankę can mean I eat porridge
• wybiera płatki can mean she chooses cereal or the cereal, depending on context

Whether something is definite or indefinite is usually understood from context, word order, or the situation.

That is one reason Polish learners from English-speaking backgrounds often feel that Polish sounds more compact.

Usually yes, in everyday translation owsianka is often given as:

• porridge
• oatmeal

The best English translation depends on region:

• British English learners may think of porridge
• American English learners may think of oatmeal

In Polish, owsianka usually means a breakfast dish made from oats, often served hot.

Yes, it can.

For example:

• Jem owsiankę z miodem na śniadanie.

This is grammatical and natural. But starting with Na śniadanie is very common when you want to set the scene first: As for breakfast...

So both work:

• Na śniadanie jem owsiankę z miodem.
• Jem owsiankę z miodem na śniadanie.

The difference is mostly about emphasis and information structure, not grammar.