A.T. Robertson Commentary Luke 1:20

"And behold, thou shalt be silent and not able to speak, until the day that these things shall come to pass, because thou believedst not my words, which shall be fulfilled in their season." — Luke 1:20 (ASV)

Thou shalt be silent (εση σιωπων). Volitive future periphrastic.

Not able to speak (μη δυναμενος λαλησα). Negative repetition of the same statement. His dumbness will continue "until" (αχρ ης ημερας) the events come to pass "because" (ανθ' ων). The words were to become reality in due season (καιρον, not χρονος, time).