W garnku są ziemniaki i cebula, bo dziś robimy prostą zupę.

W garnku means in the pot.

The preposition w usually means in, and it requires the locative case when it refers to location.
That is why garnek becomes garnku:

• garnek = pot
• w garnku = in the pot

So the sentence literally begins with the location: In the pot...

Garnku is in the locative singular.

The base form is:

• garnek = pot

After w when you mean in a place, Polish uses the locative:

• w garnku = in the pot

This kind of case change is very common in Polish after prepositions.

Because the subject is effectively plural.

The sentence says:

• ziemniaki i cebula = potatoes and onion

Even though cebula is singular, the whole combination potatoes and onion is treated as more than one thing, so Polish uses the plural form of to be:

• jest = is
• są = are

So:

• W garnku jest cebula. = There is onion in the pot.
• W garnku są ziemniaki i cebula. = There are potatoes and onion in the pot.

Polish often expresses there is / there are simply with forms of być (to be), especially when the location is stated first.

So:

• W garnku są ziemniaki.
  literally: In the pot are potatoes
  natural meaning: There are potatoes in the pot

This is a normal Polish structure. Polish does not need a separate word like English there in this kind of sentence.

Because in Polish, when talking about potatoes as food or ingredients, the plural is very common:

• ziemniak = a potato
• ziemniaki = potatoes

In cooking contexts, you would usually mention several potatoes, so ziemniaki sounds natural here.

Here cebula is part of the subject, so it stays in the nominative singular:

• cebula = onion

The sentence is naming what is in the pot:

• ziemniaki i cebula = potatoes and onion

Polish often uses the singular for ingredients like this when speaking generally. It is similar to saying potatoes and onion in English recipe-style language.

It is not cebulę, because cebulę is the accusative singular, used for a direct object.

Because zupę is the direct object of robimy (we are making), so zupa must go into the accusative case.

• nominative: prosta zupa = simple soup
• accusative: prostą zupę = a simple soup

Both the noun and the adjective change:

• zupa → zupę
• prosta → prostą

This is a very important Polish pattern: adjectives usually match the noun in case, gender, and number.

Because prostą is the accusative singular feminine form of prosta.

The noun zupa is feminine, and after robimy it is the direct object, so the adjective has to match it.

Forms:

• prosta zupa = simple soup (nominative)
• robimy prostą zupę = we are making a simple soup (accusative)

For many feminine adjectives, the ending changes like this:

• -a in nominative
• -ą in accusative

Because zupa is the thing being made, so it is the direct object of the verb robimy.

In Polish, direct objects often take the accusative case.

So:

• zupa = soup (dictionary form, nominative)
• zupę = soup (accusative)

Compare:

• To jest zupa. = This is soup.
• Robimy zupę. = We are making soup.

Bo means because.

It is very common in everyday speech and writing. It is a normal, natural word, not slang.

So:

• bo dziś robimy prostą zupę = because today we are making a simple soup

A more formal alternative is ponieważ, which also means because:

• W garnku są ziemniaki i cebula, ponieważ dziś robimy prostą zupę.

That sounds a bit more formal or written.

Dziś and dzisiaj both mean today.

• dziś is slightly shorter
• dzisiaj is also very common

In most situations, they mean the same thing and are interchangeable.

So both are natural:

• bo dziś robimy prostą zupę
• bo dzisiaj robimy prostą zupę

Polish word order is more flexible than English word order.

Starting with W garnku puts focus on the location first:

• W garnku są ziemniaki i cebula. = In the pot, there are potatoes and onion.

This sounds very natural in Polish, especially when introducing what is present somewhere.

You could also say:

• Ziemniaki i cebula są w garnku.

That is also correct, but it shifts the emphasis a bit more toward the potatoes and onion rather than the pot.

Because Polish does not have articles like English a, an, or the.

So words such as:

• garnek
• cebula
• zupa

can mean a pot / the pot, an onion / the onion, a soup / the soup, depending on context.

The listener understands from the situation what is meant.

It can mean both, depending on context.

• robimy = we do / we make / we are making

In this sentence, because of dziś and the cooking situation, the natural meaning is:

• today we are making a simple soup

Polish present tense often covers both English simple present and present continuous.

Yes, in many contexts it could.

• robimy zupę = we are making soup
• gotujemy zupę = we are cooking soup

Both can work, but they are slightly different in feel:

• robić focuses on making/preparing
• gotować focuses more on cooking

So robimy prostą zupę sounds very natural if the idea is we’re preparing a simple soup today.

Because in Polish, a comma is normally used before conjunctions like bo when they introduce a subordinate clause.

So:

• W garnku są ziemniaki i cebula, bo dziś robimy prostą zupę.

This is standard punctuation in Polish. English is less strict in some similar cases, but in Polish the comma is expected here.