Mobilna tarifa je skupa.

Because tarifa is a feminine singular noun, and Croatian adjectives must agree with the noun they describe in gender, number, and case.

So:

• mobilna tarifa = feminine singular
• masculine would be something like mobilni plan
• neuter would be mobilno ...

Here, mobilna matches tarifa.

For the same reason: it agrees with tarifa.

Even though skupa comes after je and works like expensive in English, it still changes form to match the subject:

• tarifa je skupa = feminine singular
• plan je skup = masculine singular
• rješenje je skupo = neuter singular
• tarife su skupe = plural

So Croatian predicate adjectives also show agreement.

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

So it means is.

• je = is
• su = are
• nije = is not

In this sentence, je links the subject mobilna tarifa with the adjective skupa.

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

So Mobilna tarifa je skupa can mean things like:

• The mobile tariff is expensive
• A mobile tariff is expensive
• The mobile plan is expensive

The exact interpretation depends on context.

It is in the nominative case, because it is the subject of the sentence.

• tarifa = nominative singular
• mobilna also appears in nominative singular feminine to agree with it

The adjective skupa is also in the nominative, because predicate adjectives in Croatian normally agree with the subject.

Yes. Mobilna tarifa je skupa is a normal, neutral word order.

A very common Croatian pattern is:

subject + je/su + adjective

You can change word order for emphasis, for example:

• Skupa je mobilna tarifa. = emphasis on expensive
• Mobilna je tarifa skupa. = possible, but less neutral

So the given sentence is the most straightforward version.

No, not in standard Croatian.

Unlike some languages, Croatian normally keeps the verb biti in sentences like this:

• Mobilna tarifa je skupa. = correct
• Mobilna tarifa skupa. = not standard Croatian

So je is necessary.

You replace je with nije:

• Mobilna tarifa nije skupa. = The mobile tariff is not expensive.

This is the normal way to negate this kind of sentence.

The dictionary form is usually given as the masculine singular form:

• skup = expensive

From that, the other common forms are:

• skup = masculine singular
• skupa = feminine singular
• skupo = neuter singular
• skupi / skupe / skupa = plural forms depending on gender and style

So in your sentence, skupa is just the feminine singular form of skup.

Often, yes, depending on context.

The noun tarifa can refer to a tariff, rate, or plan/pricing plan, especially in mobile-phone contexts. In everyday English, a natural translation is often mobile plan or phone plan, even if the Croatian word is tarifa.

So learners should know that the most natural English equivalent may vary by context, even though the Croatian grammar stays the same.

In the plural, both the noun and the adjectives change, and je becomes su:

• Mobilne tarife su skupe. = Mobile tariffs/plans are expensive.

Compare:

• singular: mobilna tarifa je skupa
• plural: mobilne tarife su skupe

This is a good example of how Croatian agreement works across the whole sentence.