In both these verses, the earliest text has were instead of was in the clause “I were forbidden”. The 1837 edition adopted was (“I was forbidden”), which is what we expect in standard English. For two other instances of “I were” in the earliest text, see the discussion under 3 Nephi 26:11. These instances of “I were” represent subjunctive instances of were, which was considerably more frequent in Early Modern English. Don Brugger (personal communication) provides this example from the 17th century (original accidentals retained):
The critical text will restore the earliest reading, the nonstandard use of were, in both these cases of “I were” in Mormon 1:16–17.
In the second of these two cases here in Mormon 1:16–17, Joseph Smith grammatically emended the that- clause “that I should preach unto them” to the infinitival “to preach unto them” (which is what we expect in modern English for the verb forbid ). But note that Joseph did not make this change in the first case (in verse 16). As explained under 2 Nephi 5:15, 17, the critical text will maintain or restore, as the case may be, original that- clauses, such as the two here in Mormon 1:16–17. Elsewhere the text actually prefers the that- clause as the complement for the verb forbid (along with the modal verb should):
There is one occurrence in the original text where forbidden is followed by an infinitival complement:
In each case, the critical text will follow the earliest reading, thus the two instances of the that- clause here in Mormon 1:16–17.
Summary: Restore in Mormon 1:16–17 the original two instances of the plural were in “I were forbidden”; also restore the use of the that- clause in verse 17 (“but I were forbidden that I should preach unto them”), in agreement with the use of the that- clause in verse 16 (“and I were forbidden that I should preach unto them”).