Fjármagnstekjurnar eru á skattframtalinu.

It is a compound noun plus the definite article:

• fjármagn = capital, financial capital
• tekjur = income, earnings
• -nar = the for a feminine plural noun

So fjármagnstekjurnar literally breaks down as the capital-income(s).

A useful rule in Icelandic compounds: the last part is the grammatical head. That means the whole word behaves like tekjur in gender and inflection.

That is very normal in Icelandic. Tekjur is often used in the plural where English prefers singular income.

So even if English says something like capital income, Icelandic very naturally says fjármagnstekjur. The singular tekja exists, but in this kind of financial meaning the plural is much more common.

That is also why the verb is plural: eru.

Because Icelandic usually adds the definite article to the end of the noun instead of putting a separate word before it.

Here you can see it twice:

• fjármagnstekjurnar = the capital income
• skattframtalinu = the tax return

So Icelandic often does not need a separate word like English the.

Because the subject is plural.

• er = is
• eru = are

Since fjármagnstekjurnar is plural, the sentence needs the plural form of vera (to be), which is eru.

In this sentence, it is in the nominative plural because it is the subject of the sentence.

The dictionary form is:

• tekja = singular
• tekjur = plural

Then the definite ending gives:

• fjármagnstekjurnar = the capital income / the investment income

A small detail: for many feminine nouns, nominative plural and accusative plural can look the same. Here we know it is nominative because it is the subject.

Because it is:

• dative
• singular
• definite
• and the noun is neuter

The basic noun is skattframtal (tax return). In this sentence it follows á, and á takes the dative when it means location.

So the steps are:

• skattframtal = tax return
• skattframtali = on/in a tax return (dative, indefinite)
• skattframtalinu = on/in the tax return (dative, definite)

Here á is the normal choice and is best understood as on.

With forms, lists, records, and documents, Icelandic often uses á where English also often says on:

• á eyðublaði = on a form
• á lista = on a list
• á skattframtalinu = on the tax return

Also remember this case rule:

• á
  • dative = location
• á
  • accusative = movement onto something

So here it is á skattframtalinu because the income is already listed there; there is no movement.

Yes. Icelandic word order is more flexible than English.

A very natural alternative is:

Á skattframtalinu eru fjármagnstekjurnar.

That puts more emphasis on where they are.

But Icelandic is generally a verb-second language, so when another element comes first, the finite verb still stays in second position:

• Fjármagnstekjurnar eru á skattframtalinu.
• Á skattframtalinu eru fjármagnstekjurnar.

Both are grammatical; the difference is mainly focus.

It would be:

Fjármagnstekjur eru á skattframtali.

The main changes are:

• fjármagnstekjurnar → fjármagnstekjur
• skattframtalinu → skattframtali

So you remove the attached definite endings:

• -nar
• -inu

This is a good way to see clearly how Icelandic adds the directly onto nouns.