Here we have two instances of subject-verb disagreement. In the first case (“and these things do the Spirit manifest unto me”), the plural do of the earliest text appears to be based on the preceding plural these things, even though in this sentence that noun phrase is the direct object, not the subject; the actual subject is the singular the Spirit, which follows. Thus in standard grammar we expect the -(e)s ending in modern English and the -(e)th ending in the biblical style. The singular doth was chosen for the 1907 LDS pocket edition and for the 1911 Chicago edition (probably independently), and the subsequent LDS text has maintained the doth. On the other hand, for the 1953 RLDS edition the modern form does was chosen.
In the other example, for the earliest text we get the plural verb form belong even though its associated subject is the singular every soul: “every soul which belong to the whole human family of Adam”. In this case, we may have a simple error where the third person singular ending -s or -eth was accidentally omitted from the verb belong when the text was originally dictated. (Both 𝓟 and the 1830 edition read belong, which argues that 𝓞 read the same since those two sources are both firsthand copies of 𝓞 for this part of the text.) In his editing for the 1837 edition, Joseph Smith supplied the s (giving “every soul which belongs to the whole human family of Adam”). Alternatively, the -eth ending could have been supplied (giving “every soul which belongeth to the whole human family of Adam”).
Another possibility is that the plural belong occurred in the earliest text because every soul, although singular in form, is plural in meaning. There are quite a few examples in the text where “every ” is referred to by means of a plural pronoun, as in the original text for Alma 9:28: “every man shall reap a reward of their works according to that which they have been”. More relevant here in Mormon 3:20 are three instances in the earliest text where the subject is “every ” but the associated verb is in the plural; in each case, there is some intervening text that ends in a plural form that could have triggered the plurality for the immediately following verb form:
The first example is very similar to Mormon 3:20 in its reference to belonging to the family of Adam. But here in Mormon 3:20, there is no intervening plural form such as “both men women and children” in 2 Nephi 9:21; we get simply “every soul which belong to the whole human family of Adam” in both 𝓟 and the 1830 edition.
There is some evidence that Oliver Cowdery, the presumed scribe in 𝓞, could omit the third person singular s in his manuscript work:
The example in Alma 30:16 was a permanent change. 𝓞 read “the tradition of your fathers which leads you away” (after an immediate correction of traditions to tradition). When Oliver copied the verb into 𝓟, he omitted the third person singular s from leads, giving “the tradition of your fathers which lead you away”. Yet even in this case, one could argue that the intervening plural your fathers led to the replacement of leads with lead. So actually there is not any specific evidence for permanently omitting the third-person singular s unless there is an immediately preceding plural noun.
There is also minor evidence that Oliver Cowdery could omit the -(e)th ending, although the only instance was momentary:
In this case, however, the initial repent in the if-clause was more likely due to interpreting the clause as taking the subjunctive (that is, repent is not a plural indicative form but the subjunctive infinitival form). So this is probably not an example of Oliver simply omitting a present-tense ending per se.
When we consider all other cases of “every ” where there is no intervening noun between the head noun and its associated verb, that verb is always in the singular rather than the plural. In all there are 29 instances, of which 10 are in biblical quotes. Here I list the 19 other examples:
These examples argue that the third person singular ending is missing for the verb belong in Mormon 3:20. We should also note that most of the verb forms listed above take the ending -(e)th rather than -(e)s. Excluding the cases of the be verb (is and was), there is only one example with the ending -(e)s, namely, has in Mosiah 15:13.
David Calabro suggests (personal communication) that here in Mormon 3:20 the antecedent for belong may not be the singular every soul but instead the earlier plural forms in the sentence (“and for this cause I write unto you that ye may know that ye must all stand before the judgment seat of Christ”). He also suggests that one could treat “yea every soul” as parenthetical, as it is used in the following example:
Note that the text in Mormon 8:36 has the plural have, in agreement with the earlier churches, not the immediately preceding every one. The main problem with this suggestion for Mormon 3:20 is that the phrase “yea every soul” does not appear to be at all parenthetical; the following relative clause (“which belong to the whole human family of Adam”) clearly modifies every soul, not just the people that Mormon is addressing.
If belong in 𝓞 was simply due to a scribal slip, then it seems reasonable to assume that the original text read belongs and that Oliver Cowdery simply neglected to write the s. Another possibility is that Oliver missed hearing the -eth ending in belongeth because the final th sound would have been followed by the acoustically similar voiceless t of the word to (“every soul which belongeth to the whole human family of Adam”). Since most of the verb forms listed above end in-(e)th, it seems more plausible to assume that Oliver accidentally lost the -(e)th rather than -(e)s ending as he took down Joseph Smith’s dictation, although we cannot be sure. The critical text will assume that there was some loss in the inflectional ending as Oliver took down Joseph’s dictation, probably the -eth of an original belongeth, although the -s of belongs remains a possibility. The earliest reading, belong, seems quite unlikely as the original reading.
Summary: Restore in Mormon 3:20 the original plural verb form do since it can be explained as the result of the preceding plural noun phrase, these things, even though that noun phrase is the direct object in the sentence; on the other hand, the plural verb form belong that follows the singular every soul will be considered an early error in 𝓞 for belongeth (although belongs, Joseph Smith’s emendation for the 1837 edition, is also possible).