This is the conjugation of the name Ιάσων in ancient Greek:
ὁ Ἰάσων
τοῦ Ἰάσονος
τῷ Ἰάσονι
τόν Ἰάσονα
(ὦ) Ἰάσον
Conjugation in modern Greek:
ο Ιάσονας /ο Ιάσων (ancient)
του Ιάσονα /του Ιάσονος (ancient)
τον Ιάσονα
Ιάσονα /Ιάσον (ancient)
Therefore in modern Greek the correct nominative form is Ιάσων or Ιάσονας, however the vocative form is Ιάσον or Ιάσονα.
Example:
Ο Ιάσων είναι φίλος μου. (nomonative)
or
Ο Ιάσονας είναι φίλος μου. (nomonative)
but:
Ιάσον, έλα εδω! (vocative)
or
Ιάσονα έλα εδώ! (vocative)
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.