Isaiah 48:5 (King James Bible) lest thou shouldest say mine idol hath done them
There is some evidence that Joseph Smith, when translating the Book of Mormon, could see the actual spelling of the English words (see the discussion in volume 3). In particular, the words shouldst and wouldst (as used in the current Book of Mormon text) may have originally been spelled as shouldest and wouldest. The typical pronunciation of these words for Oliver Cowdery seems to have been the one-syllable e-less pronunciation. When dictating the Isaiah 48–49 passages found in 1 Nephi 20–21, Joseph Smith may have actually seen these words with the e vowels, just as they appear in the King James Bible. And he may have dictated these words with their two-syllable pronunciations, although the evidence for that possibility (found here in 1 Nephi 20:5 and perhaps in 3 Nephi 27:3) is meager.
In 1 Nephi 20:5, Oliver Cowdery initially wrote down shouldst in 𝓞, then he erased the st at the end of the word (which shows that the correction was an immediate one). His erasure created a hole in the paper, so just after the erased st he wrote out est. Thus Oliver Cowdery corrected shouldst to shouldest. Perhaps Joseph Smith insisted on the -est spelling and could well have even pronounced the e. Nonetheless, in all subsequent uses of shouldest and wouldest in 1 Nephi 20–21 (quoting from Isaiah 48–49), Oliver Cowdery wrote only shouldst and wouldst.
In all, there are 17 occurrences of should(e)st and would(e)st in the Book of Mormon text. In the following complete list, the -est ending, whenever it occurs, is marked in bold. The five Isaiah quotations are marked with an asterisk (*), and all take the -est ending in the King James Bible itself. If the original manuscript is not extant, a minus sign (-) is placed before the item:
In the original manuscript (where extant) and in the printer’s manuscript, Oliver Cowdery always wrote shouldst and wouldst except for this one case in 1 Nephi 20:5. In the printer’s manuscript, the unknown scribe 2 wrote the -est form wouldest in 3 Nephi 27:3 (his only opportunity). But the 1830 edition, also here a firsthand copy of 𝓞, reads wouldst. The original manuscript itself is not extant for this passage, so it is difficult to tell whether 𝓞 in 3 Nephi 27:3 read wouldest or wouldst. (For discussion of this variant, see 3 Nephi 27:3.) Finally, in two cases, the 1830 typesetter set the -est ending (wouldest in 1 Nephi 20:8 and shouldest in 1 Nephi 21:6). Both cases involve quotations from Isaiah and suggest that the typesetter may have consulted his King James Bible in these two instances. Finally, the 1837 edition occasionally introduced the -est ending, but it has not persisted except in one case in the RLDS text (namely, in Alma 30:55).
Although Joseph Smith might have seen shouldest and wouldest when the text was quoting Isaiah, only once did he make sure that Oliver Cowdery wrote it down that way (namely, the first time, in 1 Nephi 20:5). For that one case, we will restore the e form. One could even argue that despite the -est spelling, this ending could nonetheless be correctly pronounced without the e. Thus maybe all cases of shouldst and wouldst could be written with -est (as in the King James Bible). The critical text, on the other hand, will take a conservative textual viewpoint and restore the -est spelling (shouldest and wouldest) only when the earliest textual evidence directly supports doing so, which is not often.
Summary: Restore the spelling shouldest in 1 Nephi 20:5 since the correction in 𝓞 suggests that at least in this case Joseph Smith made sure that Oliver Cowdery spelled out the full -est; in general, we let the earliest textual sources determine whether the ending is -st or -est; in nearly every case, the earliest textual evidence favors only the -st ending.