"Στα αλήθεια" is an expression that means really (= αληθινά in Greek). It is used as an adverb.
Τhe article στα is not typically correct in this phrase, but it is used anyway. It comes from "στα αληθινά", that is grammatically correct and means the same thing. Ie the phrase "στα αληθινά" becomes "στα αλήθεια".
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.