The 1906 LDS large-print edition replaced the present-tense know with the past-tense knew, probably because the preceding relative clause is in the past tense (“which were hard for many of my people to understand”). This change, however, has never been implemented in any subsequent LDS edition, probably because the 1906 edition has never served as a copy-text.
Here in 2 Nephi 25:1–2 the present-tense know agrees with the present-tense perfect have in the following for-clause (which begins verse 2). In other words, both for-clauses agree in having the present tense. One could therefore propose that the original text had are instead of were in the earlier relative clause and that under the inﬂuence of the preceding past-tense spake, the pasttense were was accidentally introduced. This would mean that the original text could have read “for behold Isaiah spake many things which are hard for many of my people to understand”.
Nonetheless, tense shifting within a passage does occur elsewhere in the original text, as in the following two examples (both of which refer to the Jews back at Jerusalem):
Both passages shift their time references, the first one only once, but the second one several times. In the second one, we get the simple past tense (beginning at 1 and 3) and the present-tense perfect (in 2 and in 4) as well as four modal verbs of which one refers to past time (could in 1), two to present time (must and can in 2), and one to future time (may in 4). Note, in particular, the use of the past-tense modal could early in the verse (“things that they could not understand”), but then the use of the corresponding present-tense modal can later in the verse (“things which they cannot understand”).
These examples show that the tense shifting in 2 Nephi 25:1 is possible. The relative clause can have the past-tense were followed by the present-tense know in the next clause. The critical text will therefore retain the tenses of the earliest extant source, the printer’s manuscript, in 2 Nephi 25:1.
Summary: Maintain the tense shifting in 2 Nephi 25:1, where a past-tense were is followed by a presenttense know.