At first glance, it looked like Matthew made a big mistake in his genealogy of Jesus by placing him in the line of David, since he admitted that Jesus was born of Mary, not of Joseph.  In Matthew 1:18-25, he went on to explain how Jesus was conceived and what role Joseph played in his life, which in turn explains how Jesus entered the line of David.  He also revealed the meanings of the two names of Jesus, which tell us who he is and why he came.