Then; temporal (Matthew 3:5, note). When John was preaching and baptizing. Cometh (verse 1, note). From Galilee. Mark adds, "from Nazareth of Galilee" (for this is his first historical mention of our Lord), thereby implying that our Lord had lived in Nazareth since our Matthew 2:22, etc.
In contrast to the representative teachers from Jerusalem, and the crowds both from there and from the Jordan valley (Matthew 2:5), this Stranger came from Galilee. To Jordan. It is hard to see why the Revised Version inserts "the" here and leaves the Authorized Version unaltered in Matthew 2:5.
To be baptized ( τοῦ βαπτισθῆναι); Matthew 2:13, note. By him; and no other. Not mere baptism, but baptism at the hands of John, was our Lord's motive for coming.