The word "to be combined" here means intercourse. Before they were united, Mary conceived, which is why the astonished Evangelist exclaims: "It turned out," as if speaking of something extraordinary.

But Joseph, Her husband, being righteous and not wishing to make Her public, wanted to secretly let Her go.

How was Joseph righteous? While the law commands that the adulteress should be exposed, that is, that she should be declared and punished, he intended to conceal the sin and transgress the law. The question is resolved first of all in the sense that it was through this very fact that Joseph was righteous. He did not want to be harsh, but, loving humanity in his great kindness, he shows himself above the law and lives above the commandments of the law. Then, Joseph himself knew that Mary conceived of the Holy Spirit, and therefore he did not want to expose and punish the one who conceived of the Holy Spirit, and not of an adulterer. For see what the Evangelist says: "It turned out that she was with child of the Holy Spirit." For whom did it "turn out"? For Joseph, that is, he learned that Mary conceived by the Holy Spirit. Therefore he wanted to secretly send her away, as if he did not dare to have as a wife one who had been vouchsafed such great grace.

But when he thought this, behold, an angel of the Lord appeared to him in a dream, saying:

When the righteous man hesitated, an angel appeared, teaching him what he should do. He appears to him in a dream because Joseph had strong faith. The angel spoke to the shepherds as if they were rude in reality, but to Joseph, as if he were righteous and faithful, in a dream. How could he not believe when the angel taught him what he had been talking to himself about and what he had told no one? As he pondered, but spoke to no one, an angel appeared to him. Of course, Joseph believed that this was from God, for only God knows the ineffable.

Joseph, son of David.

He called him the Son of David, reminding him of the prophecy that Christ would come from the seed of David. As the angel said this, he urged Joseph not to believe, but to think of David, who had received the promise concerning Christ.