This could be translated as "How is this possible?" Though Mary did not understand how it could happen, she did not doubt that it would happen.
## will come over you ##
This phrase and the next one are ways of saying that the Holy Spirit would supernaturally cause Mary to become pregnant even while she still remained a virgin. Make sure it is clear that there was no physical union involved; this was a miracle.
## The power of ##
This could be translated as "By his power."
## will overshadow you ##
This could be translated as "will cover you like a shadow" or "will be with you" or, "will cause this to happen." Again, make sure this does not imply any physical union.
## the holy one ##
"the holy child" or "the holy baby"
## will be called ##
This could be translated as "people will call him" or "people will acknowledge that he is."