The first 116 pages which Joseph Smith translated came from a record known as the Book of Lehi. As he wrote in the preface to the first edition of the Book of Mormon:
I would inform you that I translated, by the gift and power of God, and caused to be written, one hundred and sixteen pages, the which I took from the Book of Lehi, which was an account abridged from the plates of Lehi, by the hand of Mormon (“Preface to Book of Mormon, circa August 1829,” The Joseph Smith Papers).

The prophet Mormon had access to an enormous collection of sacred records which chronicled the entire 1,000-year history of his people. His predecessor, Ammaron, had charged him with keeping the records safe and writing about events during his lifetime (Mormon 1:2-4). At some point, he decided not only to add his own record, but also to write a history of his people, using as his source material the texts which he had in his custody. The first portion of this abridgment covered the history of his people from the time the prophet Lehi led his family out of Jerusalem (about 600 B.C.) until the time that Benjamin became king of the combined Nephite and Mulekite civilization (about 130 B.C.). However, after completing this abridgment, Mormon discovered a different set of plates which he had not previously read. These were known as “The Small Plates of Nephi,” and they covered approximately the same period of history as the portion he had already abridged (Words of Mormon 1:3-6).
For some reason he couldn’t fully understand, Mormon felt compelled to include these plates in his record. He didn’t know why, so he wrote a short editorial note explaining the redundancy to the reader and admitting that he didn’t understand why he was doing it. “I do this for a wise purpose,” he wrote, “for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me. And now, I do not know all things; but the Lord knoweth all things which are to come; wherefore, he worketh in me to do according to his will” (Words of Mormon 1:7).
Mormon’s abridgment of those years was called “The Book of Lehi,” and Joseph Smith translated it first. However, due to some errors in judgement on his part and on the part of his scribe, Martin Harris, those 116 pages were lost. The Lord instructed him not to retranslate those pages but to simply continue where he had left off. He explained that those pages had fallen into the hands of malicious people who would try to use them against Joseph. Therefore, he should simply replace that part of the history with the version contained on the plates of Nephi, which Mormon had simply inserted, unedited, into his record. (Doctrine & Covenants 10:8-45).
I’m grateful that the prophet Mormon followed the instructions he received by the Spirit of the Lord to insert the small plates of Nephi into his record. Today, I will remember that the Lord can make provision for the challenges we will face in the future, even challenges caused by our own errors. As He told Joseph Smith just after the translation of the Book of Lehi was lost, “The works, and the designs, and the purposes of God cannot be frustrated, neither can they come to naught” (Doctrine & Covenants 3:1).