It is hardly surprising that the contents of the plates impressed Mormon. Although he had the large plates record what Nephi had created, here was another record written by the founder of the Nephite nation providing a different perspective. It was perhaps the equivalent of finding a second book of Mark that covered the events of Jesus’s life from a different perspective, offering new information and new contexts. Of course, Mormon was impressed.
More difficult to understand is when Mormon says “I chose these things, to finish my record upon them, which remainder of my record I shall take from the plates of Nephi.” The problematic word is remainder. The first impression we have when reading that is that Mormon will put these plates with his record and then continue writing. That cannot be correct because Mormon has already indicated that his record is essentially finished, and he is about to give it to Moroni. What might he mean?
First, we should accept Mormon’s declaration that he is writing after the destruction of his people and that he will finish his record with these small plates. He will add them at the end of his record, and then give them to Moroni. What then of the statement that the remainder of his record would be from the large plates? The first definition of this word in Webster’s 1828 dictionary is “any thing left after the separation and removal of a part.” That might make sense if the translation is suggesting that the remainder is what is left after the small plates are removed. Thus, for a modern reader, remainder might indicate the rest.
The next problem is the verb tense of “I shall take from the plates of Nephi.” This is probably influenced by what will be the second half of Words of Mormon, discussed beginning in the next verses.