Njena nova kancelarija je najbliža stanici, ali i najskuplja u toj zgradi.

Because kancelarija is a feminine singular noun in the nominative case, and both adjectives/pronouns agree with it.

• njena = her
• nova = new
• kancelarija = office

In Serbian, words that describe a noun usually match it in:

• gender
• number
• case

So here all three are feminine singular nominative:

• njena
• nova
• kancelarija

That is why they all end the way they do.

Je is the 3rd person singular form of biti (to be), meaning is.

So:

• Njena nova kancelarija je najbliža stanici = Her new office is the closest to the station

Unlike in some languages where the present-tense to be can be dropped, in standard Serbian it is normally kept in sentences like this.

They also agree with kancelarija, which is feminine singular.

These are adjectives used after je:

• najbliža = the closest
• najskuplja = the most expensive

Because they describe kancelarija, they must match it:

• masculine: najbliži / najskuplji
• feminine: najbliža / najskuplja
• neuter: najbliže / najskuplje

So with kancelarija, the feminine forms are required.

They are superlatives.

A very common Serbian pattern is:

• positive adjective
• comparative
• superlative = naj-
  • comparative

Examples from this sentence:

1. blizu / blizak idea → bliži = closer → najbliži = closest
2. skup = expensive → skuplji = more expensive → najskuplji = most expensive

Because the noun is feminine:

• najbliži becomes najbliža
• najskuplji becomes najskuplja

So the naj- prefix is the key part meaning most / -est.

Because bliži / najbliži here takes the dative case when it means close/closer/closest to something.

So:

• stanica = base form, nominative singular
• stanici = dative singular

That is why Serbian says:

• blizu stanice = near the station
  but
• bliža stanici = closer to the station
• najbliža stanici = closest to the station

This is a very useful pattern to remember:

• blizak / bliži / najbliži + dative

Ali i means but also.

So the sentence structure is:

• ... je najbliža stanici, ali i najskuplja ...
• ... is the closest to the station, but also the most expensive ...

The word i adds emphasis to the second description. It links the two ideas smoothly:

• one positive or neutral fact: closest
• another fact, often contrasting in effect: also the most expensive

You could think of it as:

• but it is also...

Because Serbian often omits repeated words when they are understood.

Fuller version:

• Njena nova kancelarija je najbliža stanici, ali je i najskuplja u toj zgradi.

This is completely understandable and grammatical.

But the original sentence is more natural and economical:

• Njena nova kancelarija je najbliža stanici, ali i najskuplja u toj zgradi.

The second je is simply left out because it is already clear from the first part.

Because u means in, and here it expresses location, so it takes the locative case.

• ta zgrada = that building
• u toj zgradi = in that building

Both words change:

• ta → toj
• zgrada → zgradi

So this phrase tells you the area within which the office is the most expensive.

In this sentence, it most naturally applies to najskuplja:

• the most expensive in that building

So the meaning is roughly:

• Her new office is the closest to the station,
• but it is also the most expensive office in that building.

Grammatically, the phrase comes right after najskuplja, so it is most strongly connected to that part.

Serbian has no articles like English a/an/the.

So Serbian expresses ideas like the closest and the most expensive without a separate word for the. The superlative form itself does that job in context:

• najbliža = the closest
• najskuplja = the most expensive

English needs the article, but Serbian does not.

Yes, Serbian word order is more flexible than English word order, though not completely free.

The given sentence is natural and neutral:

• Njena nova kancelarija je najbliža stanici, ali i najskuplja u toj zgradi.

Other variations are possible for emphasis, for example:

• Njena nova kancelarija je u toj zgradi i najskuplja.
• Najbliža stanici je njena nova kancelarija...

But these change the focus or sound more marked. For a learner, the original version is a very good standard model.

No. Kancelarija is a common and perfectly normal word for office, especially an office room or office space.

Another very common word is:

• ured

The choice can depend on region and style. In Serbian, kancelarija is very widely used and sounds natural here.

Yes. Serbian does not need to repeat the noun if it is already clear.

So:

• najskuplja u toj zgradi

means something like:

• the most expensive one in that building

The noun kancelarija is understood from earlier in the sentence. English can do this too, but Serbian often relies on agreement and context even more smoothly.