This one is so short that I might as well quote the whole thing:
Behold, I will reveal unto you the Priesthood, by the hand of Elijah the prophet, before the coming of the great and dreadful day of the Lord.
And he shall plant in the hearts of the children the promises made to the fathers, and the hearts of the children shall turn to their fathers.
If it were not so, the whole earth would be utterly wasted at his coming.
What is the point of quoting Malachi here? This is commonly taught in Mormonism as a prophecy of the latter-day importance of genealogy and temple work. But what did it accomplish to reiterate this prophecy from Moroni's 1823 visitation? This section wasn't included in the 1833 Book of Commandments, which I suppose makes sense because we didn't know about temple ceremonies at that point. So perhaps this part of Joseph Smith's personal history was used later for section 2 to remind everyone that he received a Biblical prophecy at the beginning of his origin story that wouldn't have a full payoff until near the end of his ministry. So maybe it's a nod to his prophetic foresight.
But if that's the objective of this section's inclusion, it's strange that it seems to correct the Bible or at least to refocus the meaning of Malachi chapter 4. We're adding promises into the mix when we're talking about turning hearts and we're also tweaking the threat a little bit.
Malachi seems to make it clear that the turning of the children's hearts to the fathers and the turning of the fathers' hearts to the children will prevent the Lord smiting the earth with a dreadful curse. But this section puts the threat in a separate sentence (and even a separate verse) and makes things a little murkier.
The word "were" in the closing verse is confusing. It doesn't feel like it's looking forward in time to warn that if the heart-turning happens the smiting will be prevented. It sounds like a subjunctive form of the verb that implies something is already the case, but that if conditions were different the smiting would, hypothetically, take place. Which makes it sound not like a warning for us to turn our hearts to our fathers but as a reassurance that God has foreseen the problem and sent Elijah to turn the hearts and avert the smiting.
I'm not sure what that means, really, but it does seem to show that modern revelation adjusts and fixes established scripture. Which is fine, because that's what the church teaches, especially since the church teaches that the Bible is a little iffy when it comes to the reliability of the translation.
The problem is that the church also teaches that the Book of Mormon is not subject to the reliability issues of the Bible. Because Christ himself quotes Malachi 4:6 verbatim in 3 Nephi 25:6. So it does seem a little odd that God feels the need to rework the phrasing of Jesus's words, especially when it only muddies the cause-and-effect relationships between the different parts of his statement.
God decided to rewrite his son's sermon to make it less lucid. What are we supposed to do with this? Why did this need to be included in a set of published revelations? How is this useful to us during our quest for exaltation?