“Of the greatness of authority and of peace there will be no end.” The prophecy of the coming Messiah in Isaiah 9:1-7 blends the first and second coming of Christ and makes it seem like it will be one coming and not two. It was this kind of prophecy that led people to believe that when the Messiah came he would conquer the earth and rule it, and thus they did not understand what Jesus meant when he said he would have to suffer and die (see commentary on Luke 18:34).
“upon the throne of David and over his kingdom, [Yahweh sets His king] to establish it and to sustain it with justice and with righteousness.” The Hebrew is not a complete sentence, and the reason seems to be that this sentence relates to the promise of Yahweh to David about his dynasty, which would ultimately culminate in the Messiah but would have a number of kings along the way who would bring peace and prosperity in various measures, Hezekiah being one of those kings.
“upon the throne of David.” The Messiah was foretold to be a descendant of David and will rule “on the throne of David,” meaning, from Jerusalem where David’s throne was. When Christ sets up his kingdom on earth, he will reign from Jerusalem. [For more on Christ’s future kingdom being on earth, see Appendix 3, “Christ’s Future Kingdom on Earth.”]