Πότε is interrogative, like when BUT only in questions (When will he come = Πότε θα έρθει;).
On the other hand, όταν is conjunction for dependent sentences and it works only in declarative sentences.
Example: When I grow up, I will be a firefighter = Όταν μεγαλώσω θα γίνω πυροσβέστης. So, we can't replace one with the other.
Actually, όταν may be the answer to πότε. Example:
Πότε θα έρθεις; = When are you comming?
Όταν τελειώσω από τη δουλειά = When I finish work.
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.