Question: I was wondering, how does your translation of the book of Isaiah fit in with the Joseph Smith translation made to the Book of Isaiah?
Answer: The prophet Joseph Smith did not do a translation of the Book of Isaiah. The Book of Mormon chapters of Isaiah are taken from the King James Version, which already existed. Where Joseph Smith made changes, he mostly worked back from the English KJV to try to make it more intelligible. This can be shown from internal literary features of the Book of Isaiah.
Synonymous parallel verses in Isaiah 48:1–2, for example, reveal a double anomaly in their Book of Mormon version based on a minute scribal error in the Masoretic Text. Hebrew mimme yehudah (“from the waters of Judah”) is more correctly translated mimme‘e yehudah (“from the lineage/descendants of Judah”), which thread is continued in verse 19. The addition of “or out of the waters of baptism” in the Book of Mormon version is out of place in the Book of Isaiah, where such terminology and concepts are foreign. Here are the two verses, first the KJV, then the IIT:
“Hearken and hear this, O house of Jacob, who are called by the name of Israel, and are come forth out of the waters of Judah, or out of the waters of baptism, who swear by the name of the Lord, and make mention of the God of Israel, yet they swear not in truth nor in righteousness.”
Nevertheless, they call themselves of the holy city, but they do not stay themselves upon the God of Israel, who is the Lord of Hosts; yea, the Lord of Hosts is his name” (1 Nephi 20:1–2).
“Hear this, O house of Jacob, you who are named Israel—though you stem from the lineage of Judah—who take oaths in the name of Jehovah and invoke the God of Israel, though not in truth or in righteousness, who call yourselves of the holy city, upheld by the God of Israel, whose name is Jehovah of Hosts” (Isaiah 48:1–2).