"I have a book for her."
Translation:Έχω ένα βιβλίο για εκείνη.
For English-to-Greek translation, shouldn't "Έχω ένα βιβλίο για αυτήν." be correct?
It's the difference between 'this' and 'that'. Αυτός, αυτή, αυτό = this, εκείνος, εκείνη, εκείνο = that. Since 'her' does not include any such information, it would have to be something like "for this/that girl".
I think the question was meant, what's the difference when those words are used to mean "he, she, it" rather than "this, that".
Hmm... I edited my post as you were typing yours, let's hope it is more useful. In any case, Odontoceto can ask for a clarification! :)
The right is "Της έχω ένα βιβλίο" the same way της δίνω would be correct and not δίνω της.
Why is it wrong to say Έχω ένα βιβλίο για την? Isn't "her" a personal pronoun in accusative?
It is, but it also is the weak form and cannot stand there on its own! Weak pronoun forms exist in the safety of verbs and go straight before or after the verb. From what I can see, they only go after the verb in the imperative, see comments here or in other discussions that come up if you search for "weak AND strong AND pronoun" in the Greek discussions forum.
However there was a sentence just now "the cat is behind her" in which the weak form for "her" was used, I believe. Does it depend on the preposition itself?
It depends on whether it's a preposition or an adverb :)
"behind" is either an adverb πίσω (which can take a weak form after it) or a sort of compound preposition πίσω από which can then take a strong form.
Modern Greek has a number of such compound prepositions formed of an adverb + a preposition (usually σε "to, at, by" or από "from, of").
Though I believe that with a pronoun, it's more common to use adverb + genitive/possessive weak form of pronoun, while the compound preposition is used with nouns.
Why do I have to use the indefinite article "ένα" here? I wrote "Έχω βιβλίο για εκείνη" and it was marked wrong.