Here the 1837 edition changed the verb form witness to the third person singular form witnesses. This editing change is consistent with standard English. It could well be that the original text here actually read witnesses but that it was difficult for Oliver Cowdery to hear the third person singular /ßz/ added onto the /ßs/ of the final syllable in witness, especially given the following of /ßv/. (Or perhaps Joseph Smith himself did not pronounce the third person singular ending in this case.) An alternative explanation for witness (suggested by Alison Coutts, personal communication) is that the original text here read “which witness is of the Father and the Son”, which would have been pronounced virtually the same as “which witnesses of the Father and the Son”. Under this interpretation, witness would be a noun rather than a verb, but parallel usage in 3 Nephi 16:6 (discussed below) suggests that witness is a verb, not a noun, here in 2 Nephi 31:18.
Another possibility is that the verb form witness, but without the -es ending, is actually intended. In Early Modern English, the lexical form witness often occurred in place of witnesses. For instance, the Oxford English Dictionary cites many such examples where the plural noun form is witness rather than witnesses. Consider these examples from 1440 to 1713 listed in the OED under “uninflected plural” for definition 4a of the noun witness:
(For these examples, the spelling has been regularized.) Under the verb witness, the OED explains that witness frequently stood for the third person singular witnesses in earlier English: “In some Middle English texts, forms of the type witness (= witnesses) may be inflected forms of [the verb] witness.” One example (here with original spelling) dates from 1338: “William of Malmesbirie witnesse it in his writte.”
The Book of Mormon text has one other example involving the editing of the third person singular verb form witness to witnesses. In this instance, the 1920 LDS edition introduced the standard witnesses:
Note that in both these examples (2 Nephi 31:18 and 3 Nephi 16:6) the word witness occurs in a relative clause and immediately after the relative pronoun which.
Elsewhere in the Book of Mormon text there is one example of the alternative third person singular form, the archaic witnesseth:
But there are no other cases in the text of witness, witnesses, or witnesseth for the third person singular, which means that in the earliest text there are no examples of the third person singular form witnesses, only witnesseth (once) and witness (twice).
Turning to the noun witness, we find in the text two occurrences of the form witness that could theoretically be interpreted as the plural witnesses:
These two instances of the noun witness could actually refer to what one witnesses rather than to people who witness. Unlike the case of the verb witness, we find no specific manuscript evidence that the form witness ever replaced the plural noun witnesses. There are 20 occurrences of the plural witnesses (including two in the witness statements), but each one has consistently read as witnesses, never witness, in all the textual sources. These examples suggest that Joseph Smith and his scribes never had any particular problem mishearing or miscopying the plural noun witnesses as witness. Thus the occurrence in 2 Nephi 31:18 and 3 Nephi 16:6 of the base form witness for the third person singular witnesses seems to be intentional rather than due to error. The critical text will therefore restore these two instances of the archaic third person singular witness.
For another word ending in -ness that could stand for -nesses, see the discussion regarding the word wilderness in Alma 34:26. The plural conjuncts in that verse imply that the word wilderness actually means ‘wildernesses’:
Summary: Restore in 2 Nephi 31:18 and 3 Nephi 16:6 the archaic third person singular witness (in place of the standard witnesses).