W restauracji zamawiamy frytki z ketchupem.

Because the preposition w meaning in or at usually requires the locative case when it refers to location.

• dictionary form: restauracja = restaurant
• locative singular: restauracji = in/at the restaurant

So:

• restauracja = restaurant
• w restauracji = in the restaurant / at the restaurant

This is a very common pattern in Polish:

• w domu = at home
• w szkole = at school
• w restauracji = at/in the restaurant

Zamawiamy is the 1st person plural present tense form of zamawiać.

So it means:

• we order
• we are ordering
• sometimes we usually order

Polish present tense often covers both the simple present and the present continuous, so the exact English translation depends on context.

Breakdown:

• zamawiać = to order
• zamawiam = I order
• zamawiasz = you order
• zamawiamy = we order

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

Here, -my in zamawiamy tells you the subject is we.

So:

• zamawiamy already means we order
• my zamawiamy also means we order, but my adds emphasis, like we do it

That is why the sentence does not need my.

Because frytki means French fries, and in Polish that food is normally treated as a plural noun.

• frytka = one fry
• frytki = fries / French fries

So if you are ordering a normal portion, you usually say frytki, not frytka.

This is similar to English, where we usually say fries rather than a fry.

Frytki is the direct object of the verb zamawiamy, so it is in the accusative case.

However, for many feminine plural nouns, the accusative plural looks the same as the nominative plural.

So here:

• nominative plural: frytki
• accusative plural: frytki

That is why the form does not change visibly, even though grammatically it is the object.

Because the preposition z meaning with requires the instrumental case.

• dictionary form: ketchup
• instrumental singular: ketchupem

So:

• z ketchupem = with ketchup

This is a very common pattern:

• z herbatą = with tea
• z mamą = with mom
• z serem = with cheese
• z ketchupem = with ketchup

No. Z can mean different things depending on context.

Two very common meanings are:

1. with

   • z ketchupem = with ketchup

2. from / out of

   • z Polski = from Poland
   • z domu = from the house/home

In this sentence, z clearly means with, because it is followed by a food accompaniment: ketchupem.

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

So:

• restauracja can mean a restaurant or the restaurant
• frytki can mean fries, some fries, or the fries
• ketchup can mean ketchup or the ketchup

Context tells you which meaning is intended.

That is why W restauracji zamawiamy frytki z ketchupem has no separate words for the or some.

Yes. Polish word order is more flexible than English because the endings show grammatical relationships.

The sentence as given:

• W restauracji zamawiamy frytki z ketchupem.

is natural and neutral.

You can change the order for emphasis, for example:

• Frytki z ketchupem zamawiamy w restauracji.
  = It is fries with ketchup that we order in the restaurant.

• Zamawiamy frytki z ketchupem w restauracji.
  = We order fries with ketchup in the restaurant.

These versions are possible, but the original sounds like a straightforward, neutral statement.

It can mean either, depending on context.

Polish present tense often covers both:

• we are ordering
• we order
• we usually order

So this sentence could mean:

• right now, in a restaurant, we are ordering fries with ketchup
• or more generally, when we are in a restaurant, we order fries with ketchup

If you want to make the habitual meaning clearer, Polish often uses context words like:

• zwykle = usually
• często = often

For example:

• W restauracji zwykle zamawiamy frytki z ketchupem.

Because they are in different cases and belong to different noun patterns.

• restauracja is a feminine noun, and after w for location it goes into the locative:
  w restauracji

• ketchup is a masculine noun, and after z meaning with it goes into the instrumental:
  z ketchupem

So the endings are different because:

1. the prepositions require different cases
2. the nouns belong to different grammatical genders and declension patterns

It is a borrowed word, but yes, it is fully used in Polish. It behaves like a normal Polish noun in grammar, so it changes form by case:

• ketchup
• ketchupu
• ketchupem

You may also sometimes see the more phonetic Polish spelling keczup, but ketchup is very common and widely understood. In your sentence, z ketchupem is perfectly normal.