"How can I know for sure that what you said will happen?" Here, "know" means to learn by experience, suggesting Zechariah was asking for a sign as proof. AT: "What can you do to prove to me that this will happen?"
This can be stated in active form. AT: "God sent me to speak to you" (See: [[rc://en/ta/man/translate/figs-activepassive]])
# Behold
"Look" or "Listen." This word adds emphasis to what is said next. AT: "Pay attention to what I am about to tell you"
# silent, unable to speak
These mean the same thing, and are repeated to emphasize the completeness of his silence. AT: "completely unable to speak" or "not able to speak at all" (See: [[rc://en/ta/man/translate/figs-doublet]])