Isaiah 48:18 (King James Bible)
O that thou hadst hearkened to my commandments then had thy peace been as a river
The 1837 edition changed the plural commandments to the singular. This change was undoubtedly a typo since there is no motivation here to edit the text. Elsewhere in the Book of Mormon there are only two examples of “hearken (un)to one’s commandment(s)”; in each case the verb hearken takes the plural commandments rather than the singular:
And of course, the corresponding Isaiah passage for 1 Nephi 20:18 has the plural commandments. In accord with the early textual sources, the 1981 LDS edition restored the correct plural.
Summary: Maintain the plural commandments in 1 Nephi 20:18, the reading in the manuscripts and the King James Bible; the 1837 change to the singular was simply a typo.