Mój brat je frytki z ketchupem, ale tata mówi, że musztarda pasuje do nich lepiej niż majonez.

Because brat is a masculine singular noun, and the possessive adjective mój has to agree with it in gender, number, and case.

• mój brat = my brother
• moja siostra = my sister
• moje dziecko = my child

So here, mój is the correct masculine singular form.

Je is the 3rd person singular form of jeść (to eat), so it means he/she/it eats.

• ja jem = I eat
• ty jesz = you eat
• on/ona je = he/she eats

In this sentence, mój brat is the subject, so we need je.

Why not the others?

• jada often suggests eats regularly / tends to eat
• zjada usually means eats up / finishes eating
• je is the most neutral and natural choice here

So Mój brat je frytki simply means My brother is eating / eats fries.

Because in Polish, fries are normally treated as a plural noun, just like in English.

• frytka = one fry
• frytki = fries

After je, the direct object usually goes into the accusative case, and for this noun the accusative plural looks the same as the nominative plural:

• frytki = nominative plural
• frytki = accusative plural

So je frytki is correct.

Because the preposition z meaning with requires the instrumental case.

So:

• ketchup = base form
• z ketchupem = with ketchup

This is a very common pattern in Polish:

• z mamą = with mom
• z bratem = with brother
• z serem = with cheese
• z ketchupem = with ketchup

So the ending -em shows the instrumental case.

Yes. Ketchupem is the instrumental singular form of ketchup.

You may also see the spelling keczup in Polish, which is more fully adapted to Polish spelling. In practice:

• ketchup is very common
• keczup also exists

If you use ketchup, then after z you get:

• z ketchupem

If you use keczup, then you get:

• z keczupem

Both may be understood, but ketchup is very common in everyday writing.

Ale means but. It connects two contrasting ideas:

• Mój brat je frytki z ketchupem = My brother eats fries with ketchup
• ale tata mówi... = but dad says...

So the contrast is:

• the brother eats fries with ketchup
• the father prefers or recommends something else

This works very much like English but.

Because in Polish, once the context is clear, speakers often omit possessive words like my, your, etc.

So:

• mój tata = my dad
• tata = dad

In this sentence, tata naturally means dad / my dad, because it is understood from context.

This is very normal in Polish and often sounds more natural than repeating mój.

Że means that and introduces a subordinate clause.

So:

• tata mówi = dad says
• że musztarda pasuje... = that mustard goes / fits / works...

Together:

• tata mówi, że... = dad says that...

In English, that is often optional:

• Dad says mustard is better
• Dad says that mustard is better

In Polish, że is usually expressed here.

Because in Polish, subordinate clauses introduced by words like że are normally separated by a comma.

So:

• tata mówi, że...

This is standard Polish punctuation. It is more obligatory than in English, where comma use before that is different.

Yes, literally pasować often means to fit or to suit, but in food contexts it often means:

• goes well with
• pairs well with
• matches

So:

• musztarda pasuje do nich = mustard goes well with them

It does not mean physical fitting here. It is about taste or suitability.

Because the subject is musztarda (mustard), which is singular.

• musztarda = mustard
• pasuje = suits / goes well (3rd person singular)

Even though frytki are plural, they are not the subject. They are part of the phrase do nich (with them / to them).

So the verb agrees with musztarda, not with frytki.

Do nich here means something like with them, to them, or more naturally in English, with fries / with those fries.

The verb pasować do requires the preposition do:

• pasować do czegoś / kogoś = to suit / to go with something or someone

Since frytki are plural, the pronoun becomes:

• one = they
• nich = them (after a preposition)

So:

• pasuje do frytek = goes well with fries
• pasuje do nich = goes well with them

To avoid repetition. Polish often uses a pronoun when the noun has just been mentioned.

So instead of saying:

• musztarda pasuje do frytek lepiej niż majonez

the sentence says:

• musztarda pasuje do nich lepiej niż majonez

Both are possible. Do nich sounds natural because frytki were just mentioned.

Because the preposition do requires a specific case, and after prepositions Polish uses the corresponding pronoun form.

Here:

• do
  • nich = to them / with them

Other pronoun forms belong to other grammatical functions:

• im = to them (dative, no preposition here)
• je = them (accusative)

Since pasować do is the fixed pattern, do nich is the correct form.

It means better than mayonnaise.

• lepiej = better
• niż = than
• majonez = mayonnaise

So:

• musztarda pasuje do nich lepiej niż majonez = mustard goes better with them than mayonnaise does

In natural English, the verb at the end is often left out:

• better than mayonnaise

Polish does the same here.

Because lepiej is the normal comparative form of dobrze (well).

Like English:

• well → better

Polish:

• dobrze = well
• lepiej = better

So lepiej is the natural word to use.

Both comparison patterns exist in Polish, but they are used differently.

With comparatives, you can often compare using:

• lepszy niż... = better than...
• lepszy od... = better than...

But here the structure is built naturally with lepiej niż majonez. This is very common and straightforward.

Using niż is especially natural when you are comparing in a sentence like this:

• X pasuje lepiej niż Y

So niż is exactly what you would expect here.

Polish word order is relatively flexible, but the given order is very natural.

Standard version:

• Mój brat je frytki z ketchupem, ale tata mówi, że musztarda pasuje do nich lepiej niż majonez.

Possible variations are sometimes acceptable, for example for emphasis, but they may sound less neutral.

For instance:

• Tata mówi, że musztarda lepiej pasuje do nich niż majonez.

That is also understandable, but the original sentence is a very natural neutral sentence for everyday Polish.

It can mean either, depending on context.

Polish present tense often covers both:

• he eats
• he is eating

So Mój brat je frytki z ketchupem could mean:

• My brother eats fries with ketchup or
• My brother is eating fries with ketchup

The wider context would tell you which is intended.

Yes. Musztarda is a feminine singular noun, which is why related words would agree with it as feminine if needed.

For example:

• ta musztarda = this mustard
• dobra musztarda = good mustard

In the sentence, the verb form pasuje is singular, which matches the singular noun musztarda.

It breaks down like this:

• Mój brat = my brother
• je = eats / is eating
• frytki = fries
• z ketchupem = with ketchup

then:

• ale = but
• tata mówi = dad says
• że = that
• musztarda = mustard
• pasuje do nich = goes well with them
• lepiej niż majonez = better than mayonnaise

So the sentence is basically:

• [Statement 1], but [someone says [Statement 2]].

That is a very common and useful Polish sentence pattern.