The Book of Mormon uses plural amplification to add emphasis in the text.
Donald W. Parry explains:
In order to amplify or emphasize an idea, biblical Hebrew sometimes uses a noun in the plural when a singular is expected. The King James translators translated these Hebrew plural nouns into the English singular. In the following examples from the Old Testament the Hebrew readings appear in brackets.
– thy brother’s blood [bloods] crieth unto me from the ground (Genesis 4:10)
– and strength of salvation [salvations] (Isaiah 33:6)
– O Lord God, to whom vengeance [vengeances] belongeth (Psalms 94:1)
– Wisdom [wisdoms] crieth without; she uttereth her voice in the streets (Proverbs 1:20)
– the wicked . . . shall be brought forth to the day of wrath [wraths] (Job 21:30)
In many instances the Book of Mormon contains Hebrew-like plural nouns instead of the expected singular:
– there shall be bloodsheds (2 Nephi 1:12)
– the understandings of the children of men (Mosiah 8:20)
– great condescensions unto the children of men (Jacob 4:7)
– labor with their mights (Jacob 5:72)
– great slaughters with the sword (1 Nephi 12:2)
– there were . . . magics (Mormon 1:19)
– their cunning and their lyings (Alma 20:13)
– mine afflictions were great above all (1 Nephi 15:5)
– destructions of my people (1 Nephi 15:5)
– foolish imaginations of his heart (1 Nephi 2:11)– Donald W. Parry, “Hebraisms and Other Ancient Peculiarities in the Book of Mormon,” in Echoes and Evidences of the Book of Mormon, edited by Donald W. Parry, Daniel C. Peterson, and John W. Welch (Provo, Utah: FARMS, 2002), Chapter 7
How would Joseph Smith have known about this and how would he remember to include it (along with all the other Hebraisms) while dictating?