Ablative is the most complicated case because it consists of three old cases: instrumental, locative and ablative (separative) itself. The last function is acomplished by Genenetive in Russian. Compare prepositions: ex, ab and de with ablative in Latin and от, из, с (спрыгнуть с дерева) with genetive in Russian.
Она не читала means 'She has never started reading' Она не прочитала means 'she has not finished reading'
Может быть, перейдём на русский язык?