whats the best recourse to remembering genitive, accusative, instrumental...memorizing the phraseology or something else. i mean its tough enough trying to learn the language/words without having to wonder what the hell is what form of the sentence before and after trying to formulate ones thoughts
Hmmm... Some verbs that denote 'negativity', 'lack of' (nienawidzić = to hate, zapominać = to forget), that's for sure... But frankly, that's mostly memorizing. You can Google "Polish verbs that need Genitive" or even in Polish: "czasowniki łączące się z dopełniaczem", that may result in some lists.
"książki". If you consider the main, Nominative version, of course.
Most feminine nouns have the three following cases identical: Genitive singular, Nominative plural, Accusative plural.
As "potrzebować" (to need) takes Genitive, we see that this is Genitive singular here: I need a book.
Genitive plural (I need books) is "książek". "Potrzebuję książek".