Scholars often divide the words of the prophet into two categories. First, the prophet forth-tells. By forthtelling, the prophet applies the divine Word to the hearers’ current situation. The prophet calls people to account for their current actions and speaks to various of the day.
But secondly, the prophet foretells. This perhaps is the more common understanding of what the prophet actually does: he tells the future. The distinction can help us to understand the role of the prophet as the mediator between God and man. God chose a single man to speak to his people on his behalf. And as such, his word not only has present application for the people, but also involves telling the future of this people as a means of calling them to repentance and providing hope. The distinction shows us the richness of God’s Word and his concern for his people throughout all of history.
This is why the liberal theologians think Isaiah was written by two different people, because they can't distinguish the difference between forth and fore telling.