Here in the printer’s manuscript, Oliver Cowdery initially wrote “the Lamanites did beat them and drive them back and slay many of them”. Sometime later he changed the two infinitive verb forms drive and slay to their simple past-tense forms, drove and slew. The ink flow is considerably heavier and written somewhat awkwardly. These corrections appear to be in the hand of Oliver Cowdery and are like other corrections in the book of Mosiah that are the result of proofing 𝓟 against 𝓞 (namely, the ink flow is heavier and the supralinear correction is somewhat awkwardly inserted), as in two nearby examples:
In other words, the original manuscript, no longer extant for Mosiah 21:8, probably had the pasttense forms drove and slew, but Oliver accidentally copied them initially into 𝓟 as drive and slay; only later, when he proofed 𝓟 against 𝓞, did he correct them to drove and slew. However, there remains an alternative explanation—namely, Oliver edited the text here in 𝓟. In other words, it is possible that he originally copied drive and slay correctly from 𝓞 into 𝓟 but then later decided to edit these infinitive verb forms to their simple past-tense forms, drove and slew.
In order to evaluate these two manuscript corrections in Mosiah 21:8, let us first consider what evidence there is for replacing the simple past-tense form with the infinitive form in predicate conjuncts involving the do auxiliary. We find five passages showing this change, with responsibility for the change being assignable to Oliver Cowdery, John Gilbert (the 1830 typesetter), or Joseph Smith (in his editing for the 1837 edition):
Note, in particular, Oliver Cowdery’s correction in 1 Nephi 8:24; there he changed the past-tense partook to the infinitive form partake when he initially copied the text from 𝓞 into 𝓟. But he caught his error and with heavier ink flow corrected the partake to partook. Yet earlier, in 1 Nephi 8:22, Oliver made the same kind of mistake but did not catch it.
On the other hand, there are no examples of changes in the opposite direction (that is, from the infinitive form to the simple past-tense form when there is a preceding conjunctive predicate with the do auxiliary). Thus evidence from changes in the text argues that the infinitive forms drive and slay that Oliver initially wrote in 𝓟 are errors for the past-tense forms drove and slew, the reading of the original manuscript (no longer extant here). There seems to be little motivation for Oliver to have edited the verbs to forms that he otherwise avoided introducing into his copy work. Instead, here in Mosiah 21:8 he seems to have twice more made the same mistake (and subsequent correction) as he did in 1 Nephi 8:24—namely, replacing a simple past-tense form with its corresponding infinitive form as he copied from 𝓞 into 𝓟 (but then catching his error and correcting 𝓟 to agree with 𝓞).
As discussed under 1 Nephi 8:11, there are three possible patterns for a conjoined verb phrase when the first verb phrase has the auxiliary verb do. The auxiliary do may be repeated or, if not, the verb form can take the infinitive form or the simple past-tense form. We have a number of invariant cases in the text for the last two possibilities. Consider the following examples which involve either the verb drive or slay:
Here the past-tense usage dominates, with only one occurrence having the infinitive verb form (in Mormon 4:15). Either pattern is possible, but the clear tendency in the transmission of the text has been to replace simple past-tense forms with infinitive forms in this context. Thus the critical text will accept the two verb corrections in 𝓟 for Mosiah 21:8 as the reading of the original manuscript.
Summary: Accept in Mosiah 21:8 the two simple past-tense forms in the corrected text for 𝓟: “the Lamanites did beat them and drove them back and slew many of them”.