Κοινή [kini] "Koine" Greek and Neohellenic ("Modern" Greek) share many linguistic features. Two such features are (a) the same alphabet, and (b) the same orthography. As an example, this video shows that the vowel digraph AI,αι and epsilon E,ε stand for the same sound both in Koine and in Neohellenic. As a case in point, it uses the names Caesar and Caesaria, in Greek Καῖσαρ [kesar] and Καισάρεια [kesaria] respectively.
NOTE: ΑΙ,αι is always [e], except when the diacritic sign ( ¨ ) over iota Ϊ,ϊ is used. In such a case, ΑΪ,αϊ is pronounced [ai], each vowel belonging in a different syllable. Example: ΡΩ-ΜΑΙ-ΟΣ Ρω-μαῖ-ος [romeos] "Roman," but ΡΩ-ΜΑ-Ϊ-ΣΤΙ Ρω-μα-ϊ-στί [romaisti] "in Roman"; Ε-ΒΡΑΙ-ΟΣ Ἑ-βραῖ-ος [evreos] "Hebrew," but Ἑ-ΒΡΑ-Ϊ-ΣΤΙ Ἑ-βρα-ϊ-στί [evraisti] "in Hebrew." This video purposely bypasses these details in order to emphasize AI,αι = E,ε. —PZ