Here in Helaman 5:41, the extant Book of Mormon sources show five stages:
This stage, the earliest extant one, is the reading in 𝓟 and presumably the reading in 𝓞, no longer extant here; this stage was correctly copied into the 1830 edition.
When the text was set for the 1837 edition, the compositor’s eye skipped from the first shall to the second one; the shortened text, a fragment, makes no sense but nonetheless was copied into the subsequent 1840 edition.
The compositor for the 1841 British edition minimally emended the text here so that we get a complete sentence that answers Aminadab’s question proposed in the previous verse: “what shall we do that this cloud of darkness may be removed from overshadowing us” (Helaman 5:40). The 1841 compositor replaced the when with then and the ye with it, under the assumption that the when and ye were accidental errors that had earlier entered the text; subsequent LDS editions followed this reading until the 1920 edition. The same two changes were made in the 1858 Wright edition, set in New York City. Once more the language of the preceding verse appears to be the source for the minimal emendation in the 1858 edition, not any of the earlier editions (1841, 1849, or 1852) that had the then and it.
Here the 1874 RLDS edition followed the reading of the 1858 Wright edition except that there was a switch in the word order (probably unintentional) from it shall to shall it; the subsequent 1892 RLDS edition followed this revised reading.
Finally, the standard LDS and RLDS editions restored the original reading, as represented in the first stage. The RLDS text restored the correct reading in 1908 (probably by reference to 𝓟), the LDS text in 1920 (probably by reference to the 1830 edition).
The critical text will, of course, maintain the current reading since it is the reading in 𝓟 (the earliest extant source) and is undoubtedly the reading of the original text.
Summary: Maintain in Helaman 5:41 the reading in 𝓟, “and when ye shall do this / the cloud of darkness shall be removed from overshadowing you”; this sentence has undergone various changes in the history of the text but was restored in both the LDS and RLDS texts in early 20th-century editions.