The case Russian lost and Serbian still uses every morning

Call your mother across the room in Russian and you say Мама! - the same word you would use as the subject of a sentence. Do the same in Serbian and the word changes: Мамо!, not Мама!. For a Russian speaker several lessons into this course, that small shift is oddly disorienting, because it is one of the only places where Serbian keeps a piece of shared Slavic grammar that Russian itself let go of.

A case surviving in prayer and swearing

Old Slavic had a full vocative case, and Russian inherited it along with every other Slavic language. Modern standard Russian, though, let the case erode almost everywhere and kept only a small set of frozen exceptions: Боже! (God!), Господи! (Lord!), the literary-archaic отче (father, in prayer) and друже (friend, self-consciously old-fashioned or ironic today). None of these are productive - you cannot form a new one for a name or an ordinary noun. Everyday Russian address uses the plain nominative for absolutely everything else, „Мама, где ты?“ being representative - an ordinary name or noun never takes a special ending just because you are calling someone with it.

Colloquial Russian did invent something that looks similar - clipping a name down to its stem for informal address, Тань! for Таня, Саш! for Саша. It feels like a vocative, and it is sometimes even called one informally. But it is a much younger, purely spoken-register habit built by truncation, unrelated to the old Slavic case ending - it does not exist for common nouns, only for a shortened set of first names, and it carries none of the grammar the fossilized forms above do.

Serbian never let it go

Serbian kept the vocative fully productive. Every masculine noun, every feminine noun and every name takes a vocative ending the moment it is used for direct address, and this is ordinary, everyday, spoken-register Serbian - not a literary flourish reserved for prayer. Мамо and студенте are what an actual Serbian speaker says at breakfast, not archaisms reached for on purpose.

The Боже row is the one worth sitting with. It is identical in both languages - spelled the same, stressed the same, meaning the same exclamation. In Russian it survives as an isolated fossil, the exception every schoolchild is told to memorise precisely because nothing else works that way anymore. In Serbian it is not an exception at all: it is one entirely ordinary instance of a rule that governs every masculine noun in the language.