Fjórða bókin er mjög góð.

Breakdown of Fjórða bókin er mjög góð.

vera
to be
bókin
the book
góður
good
mjög
very
fjórði
fourth

Questions & Answers about Fjórða bókin er mjög góð.

Why is the word for "fourth" fjórða here instead of the default dictionary form fjórði?
Ordinal numbers in Icelandic change to match the gender of the noun they describe. Because bók (book) is a feminine noun, you must use the feminine nominative form fjórða instead of the masculine fjórði.
Why does bókin have the definite ending "-in" attached to it?
In Icelandic, nouns following an ordinal number generally must be in their definite form (with the "the" suffix attached). You literally say "fourth book-the" (fjórða bókin). If you just said fjórða bók, it would sound incomplete to an Icelandic speaker.
If the noun is definite (bókin), shouldn't the adjective góð have a weak ending like góða?
That weak ending rule applies when the adjective is right next to the definite noun (like in the phrase "the good book"). But here, góð comes after the verb er ("is"). Adjectives that follow a verb to describe the subject use the "strong" (default) endings. The strong feminine nominative form of "good" is simply góð.
How does this sentence show us the gender of the word bók?
Almost every word in this sentence acts as a clue! The ordinal number fjórða and the adjective góð are both specifically in their feminine forms to match the noun. Additionally, the suffix -in on bókin is the standard "the" ending for feminine nouns in the nominative case.

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