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If a Greek person wants to be rude to a stranger or to show power over them might they use the familiar form instead of the expected formal form?
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Yes, this is exactly what happened in the first sentence.
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"'Εχουμε την κόρη σας. Αν θέλετε να την ξαναδείτε, πρέπει να μας δώσετε δέκα εκατομμύρια ευρώ."
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With the phrases "Εχουμε..." and "...να μας..." he implies that the kidnappers are more than one, nothing strange here.
With the phrases "την κόρη σας", "να την ξαναδείτε" and "να μας δώσετε" he shifts from singular to plural when addressing her. However in the very first case "Ακου προσεκτικά αυτό που έχω να σου πω" he is addressing exclusively to her, since it is implied that she is the only person listening to his voice, over the phone. When he shifts to plural, it is very unlikely he does this from politeness or from mistake. He implies that the entire family will suffer the loss and they have to pay the kidnapers.
I hope this is clear.
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.