Nephi repeats that he has not taught his people how to understand Isaiah’s prophetic techniques (v. 2), although he himself possesses that knowledge. In other words, Nephi does not teach this technique because he doesn’t know it but because he chooses not to.
Furthermore, he asserts that he has personal knowledge that Isaiah’s prophecies have been fulfilled in the “lands round about,” by which he means both the Assyrian conquest of over one hundred years earlier and the more recent, and more personal, Babylonian invasion of Judah and capture of Jerusalem. Of course Jerusalem had fallen after his family left, but word of that catastrophe would have moved along the Frankincense Trail that his family traveled. He could have had confirmation from other travelers that the prophesied event had indeed taken place. He certainly knew by his father’s revelation (2 Ne. 1:3).
Having established his authority to interpret Isaiah, using Jerusalem’s destruction as a case in point, Nephi declares that he will not discuss it but will concentrate on applying Isaiah’s prophecies to the future of his own people. It is to these people and these issues that Nephi now turns.
Redaction: Verse 6 restates the theme of verses 1 and 2—Nephi’s transition to his expansion on Isaiah. By repeating his transition, Nephi signals that he has recognized his digression and now resumes his original intent.