The earliest textual sources have “remember thou” rather than the standard biblical phraseology (“rememberest thou”). The reading with the -est ending was first introduced in the 1849 LDS edition and has continued in the LDS text ever since. On the other hand, the RLDS text has retained the earliest form, remember. This example could involve an early scribal error. We should note that the word remember is hyphenated in 𝓟, not just at the end of a line but at the end of a page:
line 37, page 23 of 𝓟; line 1, page 24 of 𝓟
said the Angel had spoken these words heunto me remember thou the covenants of the Father unto the House of Israel ...
In switching to a new page, Oliver Cowdery might have accidentally replaced rememberest with the simpler and more natural remember.
Despite the possibility of scribal error here in 1 Nephi 14:8, the original text seems to have had instances where the associated verb for thou had a zero ending (that is, the verb lacked the -est ending):
In all of these cases, the text has been edited towards the expected -est ending.
It is important to recognize here that there is no manuscript variation involving mix-ups between the zero ending and the -est ending, not in any of these examples or elsewhere. This consistency implies that the original zero ending in the above examples is intentional rather than due to scribal error. The replacement of the -est ending with the zero ending shows up only once in the entire history of the transmission of the text and that error is in a single printed edition (the 1858 Wright edition), not in the manuscripts:
By contrast, there are numerous errors in the manuscripts showing the tendency of the scribes to write down the -eth ending instead of the correct -est ending. For discussion and examples of this error, see under 1 Nephi 11:2.
We should also note that here in 1 Nephi 14:8 we have a yes-no question, not the imperative. In the imperative, we expect the zero ending, as in “and to him that would borrow of thee / turn thou not away” (3 Nephi 12:42). It is possible that remember is preferred in 1 Nephi 14:8 because the archaic word order for the yes-no question matches the word order of the imperative (as in Psalm 25:7: “remember thou me for thy goodness’ sake”).
Ultimately, the lack of scribal evidence for accidentally dropping the -est ending argues that the earliest text in 1 Nephi 14:8 (“remember thou the covenants of the Father”) is intentional and should be retained in the critical text. Furthermore, the earliest text has a number of examples of thou for which the corresponding verb lacks the -est ending.
Summary: Maintain in 1 Nephi 14:8 the phraseology of the earliest text, “remember thou”; even though this form lacks the expected -est ending, evidence elsewhere in the original text and the manuscripts suggests that this usage is intended.