Ιάκοβε
In Greek there are several groups of nouns and each one of them follows its own conjugating rules.
Masculine nouns ending in " -os", in vocative case usually turn " -ος" into "-ε".
Εxample: Nominative o άνθρωπος (= man) -> Vocative: άνθρωπε
o φίλος (= friend) -> φίλε
ο Ιάκοβος -> Ιάκοβε
Masculine nouns ending in " -ας", in vocative case usually turn " -ας" into "-α" (or, more simply,in vocative case the final "s" drops off.)
Εxample:
Nominative: o γείτονας (= neighbour) -> Vocative: γείτονα
ο Κώστας -> Κώστα
About the feminine nouns:
The ones ending in "-α" or "-η" in Nominative case, usually keep the same endings in Vocative case.
Εxample:
Nominative η γυναίκα (= woman) -> Vocative: γυναίκα
η Μαρία -> Μαρία
η φίλη (= female friend) -> φίλη
η Ελένη -> Ελένη
The ones ending in "-oς" in Nominative case, in Vocative case turn "-oς" into "-ε". Εxample: Nominative η γιατρός (= the female doctor) -> Vocative: γιατρέ
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.