This is the 270th most frequent Icelandic word.
ein
"Ein" in Icelandic means "one" (feminine form).
In this sentence, 'eina' is the feminine singular accusative form of 'ein,' representing the number 'one' and modifying 'bók' in the object position.
Ég hef aðeins eina bók í hillu minni.
I have only one book on my shelf.
Here, 'ein' is an adjective meaning 'alone,' describing the subject 'Hún' in the nominative singular feminine case.
Hún er ein í herberginu.
She is alone in the room.
In this sentence, 'ein' is used in a synonymous sense with 'only one' or 'single,' emphasizing exclusivity, modifying 'sú' in nominative singular feminine.