εἶπεν δὲ Μαριὰμ πρὸς τὸν ἄγγελον· πῶς ἔσται τοῦτο, ἐπεὶ ἄνδρα οὐ γινώσκω; καὶ ἀποκριθεὶς ὁ ἄγγελος εἶπεν αὐτῇ· πνεῦμα ἅγιον ἐπελεύσεται ἐπὶ σὲ καὶ δύναμις ὑψίστου ἐπισκιάσει σοι· διὸ καὶ τὸ γεννώμενον ἅγιον κληθήσεται υἱὸς θεοῦ.
Translation (NET Bible): Mary said to the angel, “How will this be, since I have not had sexual relations with a man?” The angel replied, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be holy; he will be called the Son of God.
Brief Notes: εἶπεν is aorist indicative active, third-person singular: "he/she/it said."
The words are spoken to the angel by Μαριὰμ, which is indeclinable.
ἔσται is future middle indicative, third-person singular of εἰμί.
ἐπεὶ ἄνδρα οὐ γινώσκω: David E. Garland points to Matthew 1:25 for help with understanding this idiom. There is obviously precedent for the language in Gen. 4:1 and elsewhere.
ἐπελεύσεται is future indicative middle, third-person singular. This verb, meaning "to come upon," is used seven times in Luke-Acts..
Garland also invokes Isa. 32:15 as a parallel use of the verb; he points to Exod. 40:35 in order to elucidate the "overshadow" language.