The original text has examples of both begun and began as the past participle for the verb begin. The original Book of Mormon text frequently used the simple past-tense form of a verb for the past participle. See, for instance, the examples of “had spake” in 1 Nephi 3:30 and “had came” in 1 Nephi 5:1. (For a complete discussion, see past participle in volume 3.)
According to the earliest textual sources, the original text had at least three occurrences of the standard past participial form begun (each marked below with an asterisk) but up to six occurrences of the nonstandard past participial began:
The manuscript reading for Alma 51:12 can be interpreted either way, although in both manuscripts the second vowel looks more like an a than a u.
Here in Jacob 2:12, the 1830 typesetter replaced the standard begun with began. Otherwise, the textual tendency has been to replace the nonstandard began with begun (especially in 20thcentury editions), with the result that now the standard LDS and RLDS editions have begun in all nine cases. The critical text will restore the nonstandard cases of past participial began whenever they are supported by the earliest textual sources.
Summary: In accord with the earliest textual sources, retain the standard past participial form begun in Jacob 2:12, Alma 4:7, and Alma 38:10; in most instances, the earliest text uses the nonstandard began for the past participle of begin.