Nasze biuro jest na piątym piętrze.

Because biuro is a neuter singular noun, and nasz changes to match the noun’s gender, number, and case.

In the nominative singular:

• nasz = masculine
• nasza = feminine
• nasze = neuter

So:

• nasz dom = our house
• nasza szkoła = our school
• nasze biuro = our office

In Polish, many nouns ending in -o are neuter, and biuro is one of them.

So its basic form is:

• to biuro = this is an office

That neuter gender affects other words that go with it, such as:

• nasze biuro
• dobre biuro
• nowe biuro

In a normal full sentence like this, Polish usually keeps jest.

So:

• Nasze biuro jest na piątym piętrze. = standard and natural

Unlike in some other Slavic languages, Polish does not normally drop the present-tense form of to be in ordinary statements like this.

You may sometimes see shortened styles in signs, notes, or very informal contexts, but for learners, it is best to use jest here.

Because Polish normally uses na when talking about what floor something is on.

So Polish says:

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

Even though English often says on the fifth floor, this matches Polish well here: na piątym piętrze.

Using w would not sound correct for this meaning.

Here na expresses location, so it takes the locative case.

That is why:

• piętro becomes piętrze
• piąte becomes piątym

So:

• basic form: piąte piętro
• after na with location: na piątym piętrze

A useful contrast:

• Jesteśmy na piątym piętrze. = We are on the fifth floor. → location → locative
• Wjeżdżamy na piąte piętro. = We are going up to the fifth floor. → movement to a destination → accusative

Because piąty is an ordinal number used like an adjective, and it must agree with piętro in gender, number, and case.

The dictionary/basic form is:

• piąte piętro = fifth floor

But after na in a location phrase, we need the locative:

• na piątym piętrze

So piątym is simply the locative form agreeing with piętrze.

Because piętro is in the locative singular after na.

The nominative form is:

• piętro

The locative form is:

• na piętrze

This kind of change is normal in Polish. Many nouns change their endings depending on case.

So in this sentence:

• piętro → piętrze

Not always. This is an important cultural/language point.

In Polish building terminology:

• parter = ground floor
• pierwsze piętro = the floor above the ground floor

So piąte piętro is the fifth floor above the ground floor.

For an American English speaker, that often corresponds to what would be called the sixth floor in the US system.

Yes. Polish word order is more flexible than English word order.

The neutral version is:

• Nasze biuro jest na piątym piętrze.

You could also say:

• Na piątym piętrze jest nasze biuro.

Both are correct, but the emphasis changes:

• Nasze biuro... focuses first on our office
• Na piątym piętrze... focuses first on the location

A rough pronunciation guide is:

• Nasze ≈ NA-she
• biuro ≈ BYU-ro
• jest ≈ yest
• na ≈ nah
• piątym ≈ PYON-tym
• piętrze ≈ PYEN-tszhe

A few helpful notes:

• Polish stress is usually on the second-to-last syllable
• ą in piątym sounds roughly like a nasal on before t
• ę in piętrze is nasal too, but in normal speech it is often heard approximately as pyen-
• rz in piętrze sounds like the s in measure or zh

So the whole sentence sounds approximately like:

NA-she BYU-ro yest na PYON-tym PYEN-tszhe