Do zupy dodaję tylko jedną przyprawę, bo nie lubię, kiedy zupa jest zbyt ostra.

Do zupy means into the soup / to the soup. With verbs like dodawać (to add), Polish often uses do + Genitive to show that something is being added to something.

• dodać coś do zupy = to add something to the soup

By contrast, w zupie means in the soup and describes location, not the target of adding:

• Sól jest w zupie = The salt is in the soup

So here do zupy is the natural choice because the sentence is about putting seasoning into it.

Because the preposition do requires the Genitive case.

The noun zupa is feminine, and its singular forms include:

• Nominative: zupa
• Genitive: zupy
• Accusative: zupę

So:

• do zupy = to/into the soup

This is a very common pattern in Polish:

• do domu = to the house/home
• do szkoły = to school
• do kawy = for coffee / into coffee, depending on context

Dodaję is the 1st person singular present tense of the imperfective verb dodawać.

It can mean:

• I add
• I am adding
• I usually add

In this sentence, it most naturally expresses a habit or general preference:

• Do zupy dodaję tylko jedną przyprawę = I only add one spice to soup

If you wanted a one-time, completed action, Polish would more likely use the perfective verb dodać, for example:

• Dodałem tylko jedną przyprawę = I added only one spice

So dodaję fits well because the sentence sounds like a general rule or habit.

Because jedną przyprawę is the direct object of dodaję, and with this verb the direct object goes into the Accusative case.

The base form is:

• jedna przyprawa = one spice / one seasoning

In the Accusative singular feminine, that becomes:

• jedną przyprawę

Both words change because they must agree with each other:

• jedna → jedną
• przyprawa → przyprawę

So:

• dodaję jedną przyprawę = I add one spice

The dictionary form is przyprawa.

What you see in the sentence, przyprawę, is the Accusative singular form.

So:

• przyprawa = nominative, dictionary form
• przyprawę = accusative, used here as the object

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

• kawa → kawę
• herbata → herbatę
• zupa → zupę
• przyprawa → przyprawę

Because tylko (only) usually stands near the word or phrase it limits.

Here it limits jedną przyprawę:

• tylko jedną przyprawę = only one spice

That placement makes it clear that the speaker is not saying:

• only adds (and does nothing else), or
• adds it only to soup

but specifically:

• adds just one spice

Polish word order is flexible, but this version is very natural and clear.

Bo means because and is very common in everyday Polish.

So:

• ..., bo nie lubię... = ..., because I don’t like...

It is slightly more conversational than ponieważ, which also means because.

Compare:

• bo = common, natural in speech
• ponieważ = a bit more formal or bookish

Both are correct, but bo sounds very natural here.

This structure means I don’t like it when...

Literally:

• nie lubię = I don’t like
• kiedy zupa jest zbyt ostra = when the soup is too spicy

So the whole part:

• nie lubię, kiedy zupa jest zbyt ostra means
• I don’t like it when the soup is too spicy

This is a very common Polish pattern:

• Lubię, kiedy pada deszcz. = I like it when it rains.
• Nie lubię, kiedy ludzie krzyczą. = I don’t like it when people shout.

Yes. In many contexts, kiedy can be replaced by gdy without changing the meaning much.

So you could also say:

• ...bo nie lubię, gdy zupa jest zbyt ostra.

Both mean:

• ...because I don’t like it when the soup is too spicy.

The difference is mostly stylistic:

• kiedy = very common, neutral
• gdy = also common, sometimes slightly more literary or concise

For a learner, kiedy is a very safe and natural choice.

Because after być (to be), the adjective here agrees with the subject zupa and appears in the Nominative form.

• zupa is feminine singular
• so the adjective is ostra

Thus:

• zupa jest ostra = the soup is spicy

Not:

• zupa jest ostrą

The form ostrą is Accusative/Instrumental feminine, and it does not fit here.

Because it describes zupa, and zupa is a feminine noun.

In Polish, adjectives must agree with the noun in gender, number, and case.

So:

• ostry = masculine
• ostra = feminine
• ostre = neuter

Examples:

• ostry sos = spicy sauce
• ostra zupa = spicy soup
• ostre danie = spicy dish

Here the subject is zupa, so ostra is required.

Both can mean too spicy, but there is a small nuance.

• zbyt ostra = too spicy, a bit more neutral or formal
• za ostra = too spicy, very common in everyday speech

So:

• zupa jest zbyt ostra sounds natural and slightly more careful
• zupa jest za ostra sounds a little more conversational

Both are correct.

Polish often repeats nouns where English might use a pronoun. Repeating zupa sounds clear and natural.

• ...kiedy zupa jest zbyt ostra = clear and straightforward

You could say:

• ...kiedy jest zbyt ostra or
• ...kiedy ona jest zbyt ostra

But these are less natural in this context:

• kiedy jest zbyt ostra can work if the context is very clear
• kiedy ona jest zbyt ostra sounds more emphatic, as if you are stressing that soup

So the repetition of zupa is normal and often preferable.

Because Polish punctuation usually separates clauses more consistently than English.

Here you have:

• the main clause: Do zupy dodaję tylko jedną przyprawę
• a clause introduced by bo: bo nie lubię...
• inside that, another clause introduced by kiedy: kiedy zupa jest zbyt ostra

So the commas help mark the clause boundaries:

• ..., bo ...
• ..., kiedy ...

This is standard Polish punctuation. Even when English might be less strict, Polish normally keeps these commas.