Kroz gostujuću mrežu podatkovni promet može biti skup.

Kroz most literally means through.

In this sentence, Kroz gostujuću mrežu means something like through a roaming network or via a roaming network.

In telecom language, English would often phrase this less literally, for example:

• On a roaming network, data traffic can be expensive
• Using a roaming network, data usage can be expensive

So Croatian uses kroz where English might prefer through, via, or just a different structure altogether.

Because kroz normally requires the accusative case.

The base form is:

• gostujuća mreža = a roaming network / the roaming network

But after kroz, both the adjective and noun change into accusative singular feminine:

• gostujuću mrežu

So:

• nominative: gostujuća mreža
• accusative: gostujuću mrežu

This is a very common thing in Croatian: the preposition determines the case.

In everyday telecom usage, gostujuća mreža refers to a roaming network.

The adjective gostujuća comes from the idea of being a guest or visiting, so the image is that your phone is using a network that is not your home network.

So in context:

• gostujuća mreža = roaming network
• u gostujućoj mreži = while roaming / in a roaming network

This is a technical or semi-technical phrase, especially in mobile/telecom contexts.

Podatkovni promet means data traffic or data usage.

Breakdown:

• podatkovni = data-related
• promet = traffic, turnover, flow, depending on context

In telecom language, promet often means the amount of communication or transfer happening, such as:

• telefonski promet = telephone traffic
• internetski promet = internet traffic
• podatkovni promet = data traffic

In more natural English, depending on context, this could also be translated as:

• mobile data
• data usage
• data transfer

Yes, skup can mean different things depending on context.

Here, skup is an adjective meaning:

• expensive
• costly

Examples:

• Ovaj telefon je skup. = This phone is expensive.
• Podatkovni promet može biti skup. = Data traffic can be expensive.

So in this sentence, skup is definitely the adjective expensive, not the noun gathering or set.

Because skup agrees with promet, and promet is masculine singular.

So the adjective must also be masculine singular:

• promet → masculine singular
• skup → masculine singular

Compare:

• mreža je skupa = the network is expensive
  (mreža is feminine)
• putovanje je skupo = the trip is expensive
  (putovanje is neuter)
• promet je skup = the traffic/data usage is expensive
  (promet is masculine)

Može biti means can be or may be.

Breakdown:

• može = can / is able to / may
• biti = to be

So:

• može biti skup = can be expensive

This expresses possibility, not certainty. It does not mean that data traffic is always expensive, only that it may be.

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

So Croatian often leaves things more open, and English has to decide from context whether to use:

• a roaming network
• the roaming network
• just roaming

For example, kroz gostujuću mrežu could be rendered in English as:

• through a roaming network
• via the roaming network
• while roaming

The exact English wording depends on context, but Croatian itself does not mark that distinction with articles.

The word order is natural, but Croatian is more flexible than English.

This sentence is:

• Kroz gostujuću mrežu podatkovni promet može biti skup.

A very literal structure is:

• Through a roaming network, data traffic can be expensive.

The main idea is that the opening phrase Kroz gostujuću mrežu sets the context first.

You could also see similar Croatian word orders such as:

• Podatkovni promet kroz gostujuću mrežu može biti skup.

That would still be understandable and natural. Croatian word order often changes for emphasis, rhythm, or style more than in English.

Yes, it could be possible, but the nuance is slightly different.

• kroz gostujuću mrežu = through/via a roaming network
• u gostujućoj mreži = in a roaming network / while on a roaming network

So:

• kroz emphasizes the channel or route
• u emphasizes the state or situation

In telecom English, both might end up translated similarly depending on context, but the Croatian preposition changes the perspective.

It is grammatically singular.

That is why the verb and adjective are singular:

• promet može biti skup

Even though in English data can feel plural or mass-like, Croatian treats promet as a singular noun meaning traffic / flow / usage.

So the grammar works like this:

• subject: podatkovni promet
• verb phrase: može biti
• predicate adjective: skup

A native speaker would understand it as a warning or informational statement about mobile roaming charges.

Very natural English equivalents would be:

• Data usage on a roaming network can be expensive.
• Data traffic while roaming can be expensive.
• Using mobile data through a roaming network can be costly.

So although the Croatian sentence is fairly literal and technical, its real-world meaning is simply that roaming data may cost a lot.