Questions & Answers about Hér er bókin frá barnabarni þínu.
Why do we use þínu here instead of the basic neuter form þitt?
The preposition frá (from) always forces the words that follow it into the dative case. Because barnabarn (grandchild) is a neuter noun, we must use the neuter singular dative form of þinn (your), which is þínu.
Why does barnabarn have an -i added to the end of it?
That is the dative case ending. The word barnabarn is a neuter noun, and just like the word barn (child), it adds an -i in the dative singular. So, barnabarn becomes barnabarni.
Is it okay to change the word order to frá þínu barnabarni?
It is grammatically possible, but Icelandic normally places the possessive pronoun directly after the noun (barnabarni þínu). Putting þínu first puts a heavy emphasis on it, as if you are stressing that it is your grandchild, not someone else's.
Just a quick refresher: why is it bókin instead of just bók?
bók means "a book", while bókin means "the book". In Icelandic, the definite article ("the") attaches to the end of the noun. Because bók is a feminine noun in the nominative case here, it takes the -in ending.