Mjesečna naknada je visoka.

Because it is an adjective describing naknada, and it must agree with that noun in gender, number, and case.

• naknada is feminine
• it is singular
• here it is in the nominative

So the adjective takes the feminine singular nominative form mjesečna. The masculine form would be mjesečni, and the neuter would be mjesečno.

For the same reason: it agrees with mjesečna naknada.

Even though visoka comes after je and works like the English adjective in is high, Croatian still makes it match the subject:

• naknada = feminine singular
• therefore visoka = feminine singular

So both mjesečna and visoka are feminine singular because both relate to naknada.

Je is the 3rd person singular present tense of biti, which means to be.

So structurally the sentence is:

• Mjesečna naknada = subject
• je = is
• visoka = high

It is the normal Croatian equivalent of is in this sentence.

Normally, no in standard Croatian.

Croatian usually keeps the present-tense form of to be in sentences like this, so Mjesečna naknada visoka would sound incomplete in normal standard usage.

You may sometimes see the verb omitted in:

• headlines
• notes
• very compressed style

But for normal speech and writing, Mjesečna naknada je visoka is the correct full sentence.

It is in the nominative singular because it is the subject of the sentence.

The basic dictionary form of naknada is also nominative singular, and that is what you see here. The adjective mjesečna matches it in the same case.

Yes. After biti in a sentence like this, the predicate adjective is also in the nominative and agrees with the subject.

So:

• naknada = nominative singular feminine
• visoka = nominative singular feminine

This is why the ending looks the same as in an adjective directly before a feminine noun.

The order here is the most neutral and natural one:

Mjesečna naknada je visoka.

Croatian word order is fairly flexible, but changing it usually changes the emphasis. For example:

• Mjesečna naknada je visoka. = neutral statement
• Visoka je mjesečna naknada. = more emphasis on high
• Naknada je mjesečna. = now mjesečna means monthly as the main new information, so the meaning focus changes

So yes, other orders are possible, but they are not always equivalent in emphasis.

In Croatian, amounts such as prices, salaries, costs, fees, taxes are often described with visok / visoka / visoko, meaning high.

So visoka naknada is the natural way to say a high fee.

Velik / velika / veliko means big/large, and while it can sometimes be used for amounts, it is less idiomatic here. For a fee, visoka is the better choice.

Not always. Naknada is a fairly broad word and can mean things like:

• fee
• charge
• allowance
• compensation
• payment

The exact meaning depends on context. In this sentence, since mjesečna is attached to it, it most naturally means something like a monthly fee or monthly charge.

It is not exactly the same as:

• cijena = price
• pristojba = official fee, levy
• plaća = salary

So naknada is a flexible word with a wider range than just one English equivalent.

A simple approximate pronunciation is:

mye-sech-na nak-na-da ye vee-so-ka

A few useful sound notes:

• j is pronounced like English y
• č sounds like ch in church
• je sounds roughly like ye
• each vowel is usually pronounced clearly

So:

• mjesečna ≈ mye-sech-na
• naknada ≈ nak-na-da
• je ≈ ye
• visoka ≈ vee-so-ka

You replace je with nije:

Mjesečna naknada nije visoka.

That means The monthly fee is not high.

This is a very common pattern:

• je = is
• nije = is not

The standard yes/no question is:

Je li mjesečna naknada visoka?

That means Is the monthly fee high?

A useful pattern to remember is:

• statement: Naknada je visoka.
• question: Je li naknada visoka?

It would become:

Mjesečne naknade su visoke.

Here everything changes to plural:

• mjesečna → mjesečne
• naknada → naknade
• je → su
• visoka → visoke

This is a good example of how Croatian makes adjectives and verbs agree with the noun.