Grammatically feminine nouns ending in "-a" (a great majority of all feminine nouns, probably 85-90%) and their very straightforward rules
eg sestra = sister
Nominative : sestr{a} -- no preposition, Sentence Subject
Genitive: sestr{e} -- prepositions meaning "of", "from"
Dative/Locative: sestr{i} -- prepositions meaning "to" (Dative) or "in", "at", "about" (Locative)
Accusative: sestr{u} -- generally no preposition, Direct Object, in some very special cases, like the
verb "look", the meaning of the preposition "at", eg "gledam sestru" = "I watch [my] sister" but
"gledam u sestru" = "I am looking at [my] sister", also "zaklinjem se u sestru" = "I take an oath on
the name of [my] sister" and such.
Vocative: sestr{o} -- no preposition, appelative
Instrumental: sestr{om} -- prepositions meaning "through", "by (the means of)"
Only for personal names, Vocative is nowadays mostly equal to Nominative ie ends in "-a", traditionally it has "-o". Words meaning natural masculine gender ending in "-a" like "sudija", "Nikola" and "Ilija" fall under the same rules, for all purposes they are grammatical feminine gender but are referred to as masculinum out of common courtesy.