"She has thirteen books."
Translation:Ona ma trzynaście książek.
I was expecting książki, in the accusative. Why is it książek, in the genitive?
That's actually Accusative (not masculine-personal plural Accusative, to be precise), but the grammatical form needed by the numeral itself overrules the form of the noun in question.
So if the sentence was simple "She has books", you would be right, Ona ma książki. But adding a numeral changes everything - Ona ma:
- 1: jedną książkę
- numbers ending with 2, 3, 4: książki. dwie książki, 22 książki, 81324334 książki.
- numbers that do not end in 2/3/4: książek. 48 książek. 795 książek. 1001 książek. Exception: those that end in 12/13/14 also have 'książek': 12 książek, 914 książek, 12521513 książek.
You quite probably know this already from learning numerals in Nominative, luckily for not masculine-personal plural they look the same in Accusative.
Just another amazing rule to follow. That said, when teaching English to my students I have actually realised how hard the grammar is each language. Exceptions and rules make for fun study.