Choose the correct locative form — the case of location after u and na, and of topic after o.
Masculine & neuter: grad → gradu, selo → selu · Feminine: žena → ženi. Singular locative = singular dative in shape; the preposition tells them apart.
The contrast that matters: u školi (in school — location) vs u školu (to school — direction, accusative). Months use u + locative: u oktobru. Topic: o filmu, o Srbiji.