This is the 120th most frequent Icelandic word.
þeirri
The word "þeirri" is the dative singular feminine form of the definite article "the" or the demonstrative pronoun "that" in Icelandic.
The word 'þeirri' is used as the dative singular feminine article, referring to 'bók' (book), and indicates specificity towards the referenced book.
Ég gaf henni þeirri bók sem hún vildi.
I gave her the book she wanted.
The word 'þeirri' serves as the dative singular article describing 'ferð' (trip), emphasizing the particular trip being referred to.
Við tökum þátt í þeirri ferð sem er best.
We participate in the trip that is best.
Here, 'þeirri' is the dative singular feminine article specifying 'ákvörðunar' (decision), showing causation related to the decision made.