Matthew 7:17–18 (King James Bible) even so every good tree bringeth forth good fruit but a corrupt tree bringeth forth evil fruit a good tree cannot bring forth evil fruit neither can a corrupt tree bring forth good fruit
In the King James text for Matthew 7:18, the modal verb can first occurs negatively (as cannot); then the can is repeated, but in italics (“neither can”), which means that the repeated modal is lacking in the Greek original. The corresponding Book of Mormon reading also lacks the repeated can. Since the 1830 edition and the printer’s manuscript agree here, it is highly probable that the original manuscript also lacked the can. The question is whether the can was accidentally dropped from the original text as Joseph Smith dictated the text to the scribe, presumably Oliver Cowdery. Of course, the Book of Mormon text does agree with the original Greek, which does not repeat the modal verb can. To be sure, the Book of Mormon phraseology without can after neither does seem somewhat awkward.
If the lack of the repeated can is intended here, then we would have a parallel to an earlier instance in the Sermon on the Mount where the repeated (and italicized) you of the King James text is lacking in the Book of Mormon text:
As discussed under 3 Nephi 12:11, the critical text will maintain the reading without the repeated you after persecute (this you is italicized in the King James text). More generally, italicized King James words are frequently omitted in Book of Mormon quotations from the Bible. For some discussion of the omission of the linking verb is in Isaiah quotations, see under 2 Nephi 13:14.
Evidence elsewhere in the Book of Mormon text argues that the can is expected in 3 Nephi 14:18. For instance, in six passages we get the same basic kind of construction as in 3 Nephi 14:18, but with the repeated can (that is, “cannot ... neither can”):
Notice in particular the similarity of the metaphor in Moroni 7:11, with its parallel reference to fountains (comparable to trees) and water (comparable to fruit). Thus, one could argue that the original text in 3 Nephi 14:18 had the can but that it was accidentally lost when Joseph Smith dictated the text to his scribe.
In this case, however, the critical text will maintain the earliest extant Book of Mormon text, the reading of both 𝓟 and the 1830 edition: “a good tree cannot bring forth evil fruit / neither a corrupt tree bring forth good fruit”. In other words, the critical text will continue without the can that occurs in italics in the King James text. The similar example of the lack of the repeated you in 3 Nephi 12:11 supports this decision. In addition, we should note that the editors and typesetters for all of the published editions of the Book of Mormon have maintained the somewhat difficult reading here in 3 Nephi 14:18 without the repeated can.
Summary: Maintain in 3 Nephi 14:18 the earliest extant text, “a good tree cannot bring forth evil fruit / neither a corrupt tree bring forth good fruit”; despite the marginal difficulty and uniqueness of this reading, it is indirectly supported by usage elsewhere in the Sermon on the Mount where an italicized repeated word in the King James text is lacking in the Book of Mormon text (namely, in 3 Nephi 12:11).