Comparisons
The case Russian lost and Serbian still uses every morning
Russian technically still has a vocative case - it survives in Боже and a handful of other fossils. Serbian never let it fossilize: mama, friend or student called across a room still take a special ending, and a Russian speaker's safest instinct - reach for the plain nominative - is, with a common noun, exactly the wrong one. First names are the softer case: there the nominative is very common in speech.
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