Ako budemo dugo u minusu, kamata će opet porasti.

Because Croatian often uses future II after ako when talking about a future condition.

So:

• Ako budemo dugo u minusu ... = if we are / if we stay in the red for a long time
• main clause: kamata će opet porasti = the interest will rise again

A native English speaker may expect something like ako ćemo biti, but that is not the normal standard pattern here. In standard Croatian, future conditions are commonly built like this:

• Ako budem imao vremena, doći ću.
• Ako budemo kasnili, nazvat ćemo vas.

So budemo is not simple present here; it is part of the future-II pattern used in the if clause.

Budemo is the 1st person plural form of biti used in future II.

It tells you the subject is we, so Croatian does not need the pronoun mi.

• budem = I
• budeš = you
• bude = he/she/it
• budemo = we
• budete = you plural
• budu = they

So Ako budemo ... already means If we ...

Because Croatian usually leaves out subject pronouns unless they are needed for emphasis or contrast.

The ending -mo in budemo already shows we.

So:

• Ako budemo dugo u minusu ... = normal
• Ako mi budemo dugo u minusu ... = possible, but more emphatic, something like if we are the ones who stay in the red

This pronoun-dropping is very common in Croatian.

Literally, it means in the minus, but idiomatically it usually means:

• overdrawn
• in debt
• at a loss
• in the red

It is a very common financial expression. For example:

• Račun mi je u minusu. = My account is overdrawn.
• Firma je u minusu. = The company is operating at a loss.

In this sentence, it most naturally refers to a negative financial balance.

Yes. Dugo is an adverb meaning for a long time or long in the sense of duration.

With biti, Croatian often uses biti + dugo + somewhere/state to mean remaining in that state for a long time.

So budemo dugo u minusu is naturally understood as:

• we are in the red for a long time
• we stay in the red for a long time

It does not have to mean too long by itself; it simply refers to long duration. Any negative judgment comes from the context.

Because kamata is commonly used as a singular noun meaning interest or the interest rate/charge in a general sense.

So in this sentence, kamata is not necessarily one single interest payment. It can refer more broadly to the interest being charged.

Compare:

• Kamata je visoka. = The interest is high.
• Kamate su visoke. = The interest rates are high / the interest payments are high.

The singular is very normal here.

This is an aspect question.

• porasti = perfective, to rise/increase as a completed event
• rasti = imperfective, to be rising/increasing, or to rise in a more ongoing/general sense

In this sentence, porasti is used because the speaker means a future change will happen:

• kamata će opet porasti = the interest will go up again

If you used rasti, it would sound more like an ongoing process or repeated/general growth, not one specific increase.

Both are possible, but the structure changes slightly.

With future I, Croatian can place the clitic će before the infinitive:

• kamata će porasti

It is also possible to put the verb first, but then the infinitive usually loses its final -i:

• kamata porast će is not the normal form
• porast će kamata is correct

So:

• kamata će porasti = normal and neutral
• porast će kamata = also correct, but with different word order and emphasis

In your sentence, kamata će opet porasti is the most straightforward neutral order.

Opet means again.

So it shows that the interest has already risen before, and the speaker expects another increase.

Its position is fairly flexible, but different placement can slightly change emphasis:

• Kamata će opet porasti. = neutral
• Opet će kamata porasti. = stronger emphasis on again
• Kamata će porasti opet. = possible, but less neutral in many contexts

The version in the sentence is very natural.

Not if you mean a real future condition like this sentence does.

• Ako smo dugo u minusu means if we are / if we have been in the red for a long time, referring more to a present or already relevant situation.
• Ako budemo dugo u minusu means if we are/stay in the red for a long time in the future.

So for a future condition, budemo is the right choice.

A learner might try that because it looks closer to English if we will be, but in standard Croatian this is generally not the normal way to express a future if clause.

The standard pattern is:

• Ako budemo dugo u minusu, kamata će opet porasti.

Using ako ćemo biti sounds nonstandard or awkward in this kind of sentence. Croatian usually does not mirror English if + will here.