Ef vextirnir á þessu láni hækka aftur, verðum við að breyta áætluninni.

In Icelandic, the financial word vextir is very often plural.

So:

• vextir = interest / interest rates
• vextirnir = the interest / the interest rates

Even though English often uses a singular-looking word, Icelandic commonly treats this idea as plural. So vextirnir á þessu láni is a normal way to say the interest rates on this loan or simply the interest on this loan.

The ending -nir is the suffixed definite article, meaning the.

So:

• vextir = interest rates
• vextirnir = the interest rates

In Icelandic, the is often attached to the end of the noun instead of standing as a separate word.

Here, vextirnir is also the subject of hækka, so it is in the nominative plural.

Because the preposition á takes the dative here.

A very useful rule is:

• á + dative for a static position/state
• á + accusative for movement onto something

In this sentence, there is no movement. It means on this loan in a financial sense, so Icelandic uses the dative:

• þessu = dative singular of þessi
• láni = dative singular of lán

So á þessu láni is the correct form.

Because Icelandic normally does not use both a demonstrative like this and the suffixed definite article on the same noun in standard usage.

So:

• lánið = the loan
• þetta lán = this loan
• þessu láni = on this loan / with this loan depending on context

Using þessu láninu would usually sound like this the loan, which standard Icelandic avoids.

Because the subject is plural: vextirnir.

The verb hækka here is in the 3rd person plural present:

• hann hækkar = it rises
• þeir hækka = they rise

Since vextirnir = the interest rates is plural, the correct verb form is hækka.

This is because of Icelandic V2 word order in main clauses.

When another element comes first in the sentence, the finite verb usually comes next. Here, the whole if-clause comes first:

• Ef vextirnir á þessu láni hækka aftur = fronted subordinate clause

Then the main clause begins, and Icelandic puts the finite verb first:

• verðum við að breyta áætluninni

So the pattern is basically:

• If-clause, then
• finite verb + subject

That is why you get verðum við, not við verðum.

Verða að is a very common expression meaning have to or must.

So:

• við verðum að breyta = we have to change / we must change
• in this conditional sentence, it naturally comes out as we will have to change

It is best learned as a fixed expression:

• verða að + infinitive

Here the infinitive is breyta.

Because the verb breyta governs the dative.

So Icelandic says literally:

• breyta einhverju = change something

with something in the dative.

That is why the noun is:

• áætluninni = dative singular definite of áætlun

So the structure is:

• að breyta áætluninni = to change the plan

This is something learners usually just have to memorize with the verb:

• breyta + dative

Yes:

• áætlun = plan
• -inni = the definite article ending here, in the dative singular

So:

• áætlun = a plan
• áætluninni = the plan (in the dative case)

It is dative because of breyta, not because of the article itself.

Because Icelandic often uses the present tense to talk about future situations, especially in conditionals.

Here:

• hækka is present
• verðum is present

But the meaning is future because the whole sentence is about what will happen if something happens later.

This is very normal in Icelandic. English often does something similar in if-clauses:

• If rates go up again, we’ll have to change the plan.

So Icelandic is not being unusual here.

Here aftur means again.

So:

• hækka aftur = rise again / go up again

Its position is very natural in Icelandic. Adverbs like aftur often come after the verb in a neutral sentence.

So the phrase:

• vextirnir ... hækka aftur

simply means:

• the interest rates rise again

Because the sentence begins with a subordinate clause:

• Ef vextirnir á þessu láni hækka aftur,

and then moves to the main clause:

• verðum við að breyta áætluninni.

In standard Icelandic writing, a subordinate clause placed before the main clause is commonly separated by a comma. So the comma helps mark that boundary clearly.