In a past Maxwell Institute publication, Daniel Peterson made a great point about the nature of the language in the Book of Mormon that seems to defy theories of Joseph Smith as its author. See Daniel C. Peterson, “Mormonism as a Restoration,” FARMS Review 18/1 (2006): 389–417. The article is no longer available at the Maxwell Institute’s website, but is at the Scholar’s Archive at BYU (PDF form only — the numerous HTML-format articles at the Maxwell Institute did not survive the recent changes there).
A number of details from the Book of Mormon text appear to support a view of the book as a rather literal translation from an ancient document. In an ancient Hebrew idiom, for example, arrows are “thrown” (see, for example, Alma 49:22). Also, just as in ancient Hebrew and other Semitic languages, in a construction known as a “cognate accusative,” the word denoting the object of a verb is sometimes derived from the same root as the verb itself. “Behold,” says the prophet Lehi, “I have dreamed a dream.” Similarly, the (to us) redundant that in such expressions as “because that they are redeemed from the fall” and “because that my heart is broken” is a Hebraism (see, respectively, 2 Nephi 2:26 and 4:32).
But some Hebrew constructions that appeared in the first (1830) edition of the Book of Mormon have been erased from later printings, in a bid to make the book read more smoothly as English. One striking example of this involves a series of conditional sentences in Helaman 12:13–21. Such sentences, in English, typically feature an if-clause (either using the word if itself, or something equivalent), which expresses a hypothetical condition, and a result clause that describes what will occur if the hypothetical condition comes about. For example, “If you don’t study, you will fail.” The result clause may contain a word such as then, but commonly does not. By contrast, the result clause of a conditional sentence in ancient Hebrew can be introduced by the word wa (and), so that the sentence takes what might be termed an if-and form. The occurrence of if-and conditionals in the 1830 Book of Mormon seems to indicate that it did not originate in the mind of a native English-speaker, but is a quite literal translation from a Hebrew original:
13. yea and if he saith unto the earth move and it is moved.
14. yea if he say unto the earth thou shalt go back that it lengthen out the day for many hours and it is done.
16. and behold also if he saith unto the waters of the great deep be thou dried up and it is done.
17. behold if he saith unto this mountain be thou raised up and come over and fall upon that city that it be buried up and behold it is done.
19. and if the Lord shall say be thou accursed that no man shall find thee from this time henceforth and forever and behold no man getteth it henceforth and forever.
20. and behold if the Lord shall say unto a man because of thine iniquities thou shalt be accursed forever and it shall be done.
21. and if the Lord shall say because of thine iniquities thou shalt be cut off from my presence and he will cause that it shall be so. (Helaman 12:13–14, 16–17, 19–21, 1830 edition)
4. and if ye shall ask with a sincere heart with real intent having faith in Christ and he will manifest the truth of it unto you by the power of the Holy Ghost. (Moroni 10:4, 1830 edition)
It is difficult to imagine a native speaker of English (such as Joseph Smith, though poorly educated at the time, indisputably was) producing such sentences. Yet they represent perfectly acceptable Hebrew.
Since that time, further research from Royal Skousen and Stanford Carmack suggests that there is a persistent thread of Early Modern English in the text, some of which has been edited out to make the grammar more standard for our era. Whatever the cause for this, it’s unreasonable to believe it was just Joseph’s Yankee dialect at work. It’s a fascinating complex of details about the translation of the Book of Mormon, a book that upon closer examination readily overturns many of the theories that have been offered to explain it as a modern product of Joseph Smith’s environment.
On the other hand, the evidences mentioned above can be over-simplified. Not everything is Early Modern English or Hebraisms. It’s a complex text.
Update, Aug. 6, 2019: Perhaps more interesting or more persuasive than the interesting oddities of grammar is the existence of legitimate Old World names in the many Book of Mormon names that are not found directly in the KJV. For starters, consider this brief 2013 article from Stephen D. Ricks at The Interpreter: “Some Notes on Book of Mormon Names.” But there is much more that could be said. Also consider Matthew Bowen’s many articles at The Interpreter on the intelligent use of wordplays associated with Old World names in the Book of Mormon. You can see some information about numerous other Book of Mormon names in the Book of Mormon Onomasticon at BYU.edu, though in some cases there may be recent discoveries that have not yet have been entered into the data presented there.