Breakdown of Fjármagnstekjurnar eru á skattframtalinu.
Questions & Answers about Fjármagnstekjurnar eru á skattframtalinu.
It is a compound noun plus the definite article:
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.