καταρχήν καταρχάς

Καλησπέρα, τὴν ἀρχήν is the accusative singular of ἀρχήχή while τὰς ἀρχὰς is the accusative plural. Κατά requires accusative when it is not used in the sense of "against", this I understand. I also understand that αρχή, in Modern Greek, has a different meaning, e.g. start, authority, principle...