"Libro" is the ablative sing. and also the dative sing. for "liber", which, first, confused me a bit. If it can help someone who's a beginner:
A place that is not a geographical place:
in + ablative = in libro.
In libro scribo. In libris veritas.
Dative, if I mean "for the book" or "to the book" (as you give something to someone, I rated it: I gave a star to this book. ). I could die for this book.
Accusative, when the book is the direct object (without any preposition, receiving directly the action of the verb) Ex: I see the sky. I take the book.
One question is whether book (liber, libri, m.) means "a book" in the modern sense (pages = leaves sewn together inside a rigid cover), or a scroll with papyrus that's rolled open or closed. I don't know a lot about material culture; but I believe that the scroll (volumen, voluminis, n.--but liber, I believe, could also be used for volumen, esp. before the real "book" or codex was invented) had a little tag hanging from it, on which the book's name or the author's name could be written. One meaning of index, indicis, m., is this tag. "I write on the (cover/tag) of the book", in any case, will have libri, the genitive form of book.
In the book. Write on the book, is rather on the cover, and in the book, is inside the book, on the pages.
As a book is considered as something that has an exterior part (on the book), and an interior part (in the book = on the pages), and it's "on the pages", because it's not the case for pages.
Shouldn't it be the accusative, since there is a "movement" from the hand to the book? In German, anyways, it would be the accusative - there is the same distinction between accusative and dative in German as between accusative and ablative in Latin, for these kind of local prepositions.
No. The accusitive (object) is what actually receives the action.
It's not revealed in the sentence, but it could be words, a story, a novel, etc.
I write stories.
Stories is the object and would be accusitive.
I write stories in a book.
Book is the object of the preposition, not the object of the verb in the sentence. Which I guess is ablative (I'm learning too).
The sentence we are given is essentially
I write __ in a book.
So there is no direct object, and no accusitive.