In nominative singular nouns of neuter gender (imenice srednjeg roda) end with -o or -e. The same is with adjectives. They end with -o or -e. But there aren't or at least I don't know clear rules for choosing ending. Examples: hladno more (hot sea); toplo more (warm sea); toplo ljeto (warm summer); visoko stablo (tall tree). TOPLO LJETO Singular NOMINATIV: toplO ljetO GENITIV: toploG(A) ljetA DATIV: toploM(e)/(u) ljetU AKUZATIV: toplO ljetO VOKATIV: toplO ljetO LOKATIV: toploM(e)/(u) ljetU INSTRUMENTAL: toplIM ljetoM plural N: toplA ljetA G: toplIH ljetA D: toplIM ljetIMA A: toplA ljetA V: toplA ljetA L: toplIM ljetIMA I: toplIM ljetIMA This is a scheme but Croatian has enough exceptions. Now about possessive adjectives. They end with -ski, -ški, -ji, -nji, -šnji, -ov, -ev, -in, -jev, -ljev,... These endings must be changed into propriate way so they can be agreed with neuter nouns. Examples: hrvatsko jezero (Croatian lake), prijateljevo stablo (tree of friend; friend's tree).