Vocative case/Talk

I'm not completely familiar with the technical jargon of verb tenses in English (sorry), but what do you mean when you say that, in English, the vocative case isn't marked? Reason I ask is because of the form "Sing in me, O muse", which is vocative. I don't believe "O" has any other function in English, so is that not a mark? -- PaulDrye

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I couldn't think of any other use for O either but you can make English vocative case sentences without O or any other mark. --rmhermen