Of what does Nephi speak when he says: "in the which I know that no man can err?" Is it that no man may err in Nephi's prophecy, or in his plainness? Ultimately, the goal is understanding the prophecy, but in this case it is probably his plainness in writing to which Nephi is referring. This contrasts with the "manner of prophesying of the Jews" which he understands has prevented some from understanding Isaiah.
Regardless of the understanding of Nephi's coming prophecy on Isaiah's themes, Nephi indicates that when those predictions come to pass, then shall we know completely. Such is the real nature of prophetic utterances. Even the clearest do not necessarily show us, with precision, times and places. Nevertheless, when the prophecy is fulfilled, then we are able to see how that prophecy fit into the events.