W magazynie na parterze stoją pudła z papierem do drukarki.

Because both follow prepositions that describe a location, and in Polish that usually means the locative case.

• w magazynie = in the warehouse
• na parterze = on the ground floor

The basic forms are:

• magazyn
• parter

After w and na when they mean where? rather than to where?, Polish uses the locative:

• magazyn → w magazynie
• parter → na parterze

So this is normal case grammar, not a special irregular sentence pattern.

Because Polish normally uses na with floors of a building:

• na parterze = on the ground floor
• na pierwszym piętrze = on the first floor
• na drugim piętrze = on the second floor

This is just the standard idiomatic choice in Polish. English speakers often expect w because English says in the building, but with floors Polish typically says na.

Polish often prefers a position verb where English would simply use to be.

• stoją = are standing
• leżą = are lying
• wiszą = are hanging

So stoją pudła suggests the boxes are there in an upright/placed position. It sounds more natural and vivid than just są.

You could say W magazynie na parterze są pudła..., and it would be understandable, but stoją is more idiomatic here.

Because Polish word order is flexible, and this order is very natural when you are introducing what is present in a place.

• W magazynie na parterze stoją pudła...
  = In the warehouse on the ground floor, there are boxes...

This structure puts the location first, then says what is there. It is common in descriptions.

If you said:

• Pudła stoją w magazynie na parterze

that would also be correct, but it shifts the emphasis more toward the boxes as the topic.

Pudła is the nominative plural of pudło.

• singular: pudło = box
• plural: pudła = boxes

It is nominative because pudła is the subject of the verb stoją.

So the basic structure is:

• pudła stoją = the boxes are standing

Even though the subject comes after the verb in this sentence, it is still the subject, so nominative is required.

Because z can take different cases with different meanings.

Here, z papierem uses the instrumental case and means:

• with paper
• containing paper
• together with paper

So pudła z papierem means boxes with paper inside / boxes containing paper.

By contrast:

• z papieru uses the genitive case
• it means made of paper

So:

• pudła z papierem = boxes with paper
• pudła z papieru = boxes made of paper

That is an important distinction.

It is one noun phrase inside the larger phrase z papierem do drukarki.

Break it down like this:

• papier = paper
• papier do drukarki = paper for a printer / printer paper
• z papierem do drukarki = with printer paper

So do drukarki belongs with papierem, not directly with pudła.

In other words, these are:

• boxes
• with paper
• specifically paper for a printer

After the preposition do, Polish uses the genitive case.

• basic form: drukarka = printer
• genitive singular: drukarki

So:

• do drukarki = for the printer / to the printer

In the phrase papier do drukarki, the meaning is paper intended for printer use, often best translated as printer paper.

Compare:

• papier do drukarki = paper for the printer
• papier z drukarki = paper from the printer

Polish has no articles like a/an/the.

So words like:

• magazynie
• pudła
• drukarki

do not by themselves tell you whether something is definite or indefinite. That is understood from context.

Depending on context, the sentence could be interpreted as:

• In the warehouse on the ground floor, there are boxes with printer paper.
• In the warehouse on the ground floor, the boxes with printer paper are standing there.

Usually the broader situation tells you which reading is intended.

It is really one location described in two layers:

• w magazynie = in the warehouse
• na parterze = on the ground floor

So together they mean something like:

• in the warehouse that is on the ground floor
• or in the warehouse, on the ground floor

Polish often stacks location phrases like this very naturally. English does something similar, but the Polish structure may feel more compact.

Yes, that is a very literal rendering of the structure:

• W magazynie na parterze = In the warehouse on the ground floor
• stoją = stand / are standing
• pudła = boxes
• z papierem do drukarki = with printer paper

But in natural English, you would usually say something like:

• There are boxes with printer paper in the warehouse on the ground floor.
• Boxes with printer paper are standing in the warehouse on the ground floor.
  (more literal, less natural in everyday English)

So the Polish sentence is perfectly normal, even if the closest word-for-word English version sounds a bit stiff.