Isaiah does not always make a clear distinction between the personal “I” that refers to Isaiah the man and the prophetic “I” in which he is Yahweh’s mouthpiece. In this verse, the “I” is prophetic, as the events indicated describe Yahweh’s care for and working with his people.
The referent of “former things” can either be his harsh summons to an Israel in need of repentance or a generic set of previous prophecies. Either interpretation is appropriate. It describes two important facets of prophecy: first, that prophecies come from Yahweh, and second, that they are fulfilled. I suspect that “I did show them suddenly” means that we are often unaware that prophecy has been fulfilled until after the fact, no matter how long the prophecy has been known.
Application to Nephi’s audience: This powerful point is directly relevant, not only to Isaiah’s original audience, but to Nephi’s as well. This passage sets up a paradigm of prophecy/fulfillment, in which Yahweh indicates his willingness to guide his children and then to make good on his words. The Lehites have already experienced the critical role of prophecy and its fulfillment many times before they reached the New World.