Hello
"Πού ήσουνα;" means "where were you?". It does not mean "where are you?".
The past endings -α , -ες , -ε etc apply to regular verbs of groups Α and Β in active voice.
The verb είμαι = I am is an irregular verb. It does not belong to one of these groups and its conjugation is different.
The Past tense of είμαι:
ήμουν
ήσουν
ήταν
ήμασταν
ήσασταν
ήταν
"ήσουνα" is an oral variation of "ήσουν". Τhe α in the end is put for phonetic reasons, when the following word starts with a consonant.
Example:
Πού ήσουν εχθές; (the word after ήσουν starts with a vowel)
Πού ήσουνα προχθες; (the word after ήσουνα starts with a consonant)
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.