Thou shalt be silent (esēi siōpōn). Volitive future periphrastic.
Not able to speak (mē dunamenos lalēsai). Negative repetition of the same statement. His dumbness will continue “until” (achri hēs hēmeras) the events come to pass “because” (anth' hōn). The words were to become reality in due season (kairon, not chronos, time).