Dodatni paket je skup.

• dodatni = an adjective, describing paket
• paket = a noun, meaning package / bundle / pack
• je = the 3rd person singular of biti (to be) = is
• skup = an adjective here, meaning expensive

So the structure is basically:

adjective + noun + is + adjective
= additional package is expensive

Because dodatni has to agree with paket.

The noun paket is:

• masculine
• singular
• nominative (the basic dictionary/subject form)

So the adjective also appears in the masculine singular nominative form:

• dodatni paket

In Croatian, adjectives change form to match the noun they describe.

Because paket is the subject of the sentence.

In Dodatni paket je skup, the sentence is saying something about the package:

• The additional package = subject
• is expensive = what is being said about it

Subjects are normally in the nominative case in Croatian.

This is a very common learner question.

Here skup is a predicate adjective: it comes after je and describes the subject:

• Paket je skup. = The package is expensive.

In this position, Croatian often uses the short/indefinite form of certain adjectives:

• skup = expensive

You may also see skupi in other contexts, especially before a noun:

• skupi paket = the expensive package / an expensive package

So:

• skup is very natural after je
• skupi is more typical directly before a noun

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

So paket can mean:

• a package
• the package

The exact meaning depends on context.

That is why Dodatni paket je skup can be understood as:

• The additional package is expensive or
• An additional package is expensive

depending on the situation.

Je is the present-tense form of biti (to be) for:

• he/she/it is

Here it matches paket, which is grammatically singular, so:

• paket je = the package is

Very common examples:

• Auto je novo. = The car is new.
• Hotel je skup. = The hotel is expensive.

Yes, Croatian word order is more flexible than English, but not every order sounds equally neutral.

The most neutral version is:

• Dodatni paket je skup.

You may also hear:

• Paket je skup. = The package is expensive.
• Skup je dodatni paket. = marked/emphatic, not the usual basic order

For learners, the safest choice is to keep the neutral order: adjective + noun + je + adjective

A rough English-style guide:

• Dodatni ≈ doh-DAHT-nee
• paket ≈ pah-KET
• je ≈ yeh
• skup ≈ skoop

A few helpful notes:

• j in Croatian is pronounced like English y in yes
• u is pronounced like oo in food
• stress can vary somewhat by dialect, but the above guide is close enough for a learner

No. Skup can have different meanings depending on context.

As an adjective, it commonly means:

• expensive

But skup can also be a noun meaning something like:

• gathering
• assembly
• set (in mathematics, depending on context)

In Dodatni paket je skup, it is clearly an adjective meaning expensive, because it comes after je and describes paket.

In a normal present-tense statement like this, no.

Croatian does sometimes omit to be in certain styles, headlines, or special expressions, but in a standard sentence you should say:

• Dodatni paket je skup.

Without je, it would sound incomplete in ordinary usage.

Not really a fixed phrase, but it is a very natural combination.

dodatni means:

• additional
• extra

So dodatni paket can mean:

• an additional package
• an extra package
• an add-on package / extra package, depending on context

For example, in mobile plans, internet plans, or service offers, dodatni paket often means an extra package or add-on package.

The adjectives would change to match the gender of the noun.

For example:

Feminine:

• Dodatna usluga je skupa.
  = The additional service is expensive.

Neuter:

• Dodatno osiguranje je skupo.
  = The additional insurance is expensive.

Compare the adjective endings:

• masculine: dodatni, skup
• feminine: dodatna, skupa
• neuter: dodatno, skupo

This agreement is a key feature of Croatian grammar.