The original manuscript here reads in the plural as my pains. In his copying into the printer’s manuscript, Oliver Cowdery accidentally changed this phrase into the singular my pain. The singular pain has been retained in all the printed editions. Earlier in this chapter, Oliver made the same error initially in 𝓞; but in that case he virtually immediately corrected pain to pains by inserting the plural s. The s is written inline but somewhat above the line, and there is no change in the level of ink flow for the inserted s:
The plural usage is correct here in both Alma 36:16 and Alma 36:20. In fact, pains is the consistent reading of the manuscripts elsewhere in this chapter for every instance used to describe Alma’s suffering:
In the last example, the earliest text reads “as was my pains”. The 1852 LDS edition, in its first printing, reads “as was my pain”. The singular here probably resulted from the preceding singular verb form was. The second 1852 printing, following the 1840 edition, restored the plural pains (“as was my pains”). Finally, the 1911 LDS edition removed the subject-verb disagreement in Alma 36:21 by replacing the was with were (“as were my pains”). The critical text will, of course, restore the original “as was my pains”. For discussion regarding the use of was with plural subjects, see under 1 Nephi 4:4 (and, more generally, under subject-verb agreement in volume 3).
Summary: In accord with the reading of the original manuscript, restore the plural pains in Alma 36:20 (“as was my pains”); this reading is consistent with Alma’s use of the plural pains throughout this chapter to refer to his suffering; in Alma 36:21, the singular verb form was should also be restored (thus “as was my pains”, the same as in verse 20).