“behold.” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. Here in Matthew 2:13, “take note” or something similar catches the meaning. See commentary on Matthew 1:20.
“the Lord” “Yahweh” is the personal name of God, and a rabbinic abbreviation for it appears in the Hebrew manuscript of Matthew as well as in the verses of the Old Testament that Matthew quoted. There is evidence that Matthew wrote his Gospel in Hebrew and used the name Yahweh (see commentary on Matthew 3:3).
“Get up!... take the child and his mother… stay.” These exact words are used to describe what Joseph did in response to the command from the angel of the Lord. The angel says “get up,” so Joseph “got up” (Matt. 2:14); the angel says, “take the child and his mother,” so Joseph “took the child and his mother” (Matt. 2:14); the angel says “stay there until,” so Joseph “stays there until” (Matt. 2:15). This parallelism highlights Joseph’s obedience to the word of the Lord, by describing what Joseph did with the same words the angel used in his command. The same parallelism occurs in Matt. 2:20-21.
“stay there until I tell you.” For the word stay, the angel uses the verb “to be,” eimi (#1510 εἰμί), with the sense of “remain” or “stay;” Joseph is told to “be there” in Egypt until the angel tells him differently. Then in Matthew 2:15, we are told that Joseph “was there”—using the same verb and word for “there.” Since we do not know when the Magi arrived in Bethlehem, we do not know how long Joseph and Mary stayed in Egypt. However, it seems like it would not have been too long—months, not years.