Breakdown of Spurningin er um höfuðborgina, er það ekki?
vera
to be
það
it / there (existential) / the one (n)
ekki
not
spurningin
the question (f)
Questions & Answers about Spurningin er um höfuðborgina, er það ekki?
Does the tag er það ekki change if we use a different verb or a plural subject?
No, and that is the great thing about Icelandic question tags! While English changes the tag to match the sentence (isn't she, don't they, haven't we), Icelandic generally uses the fixed phrase er það ekki? (literally: is it not?) for almost all confirmation questions.
Why does the sentence use höfuðborgina instead of the nominative form höfuðborgin?
It is in the accusative case because it directly follows the preposition um (about), which always forces the noun into the accusative case. Because höfuðborg is a feminine noun, its attached definite article changes from the nominative -in to the accusative -ina.
Is höfuðborg a compound word?
Why does the word spurningin end in -in?
This is the attached definite article, meaning 'the'. Since spurning (question) is a feminine noun and acts as the subject of the sentence (nominative case), it takes the feminine nominative suffix -in.