What do we learn from "v'Heshiv Lev Avos Al Banim v'Lev Banim Lev Al Avosam"?
Rashi citing R. Menachem: Eliyahu will tell the sons to tell their fathers to seize Hashem's ways. Similarly, he will tell fathers to tell their sons to repent.
Rashi citing Eduyos 8:7: He will make Shalom in the world.
Radak: He will return the fathers and sons together.
Radak (23): Eliyahu will warn the fathers and sons to repent with all their hearts. Those who repent will be saved from the day of judgment.
Malbim: Eliyahu will make all repent. Sons who were led away from their fathers' Torah, he will return their hearts to the creed and Torah of their fathers. This will return the fathers' hearts to their sons.
Why does it say "Pen Avo v'Hikeisi"?
Radak: He will warn them to repent, lest Hashem strike the land. Those who are not careful will be eradicated in Midbar Amim, or on the day of judgment in Eretz Yisrael. Those who are careful will shine like the sky, and those who bring merit to the Rabim will be like stars, forever.
Malbim: This is why Hashem will send Eliyahu first to straighten the nation, lest He come suddenly and strike the land. First He will send Mal'ach ha'Bris to make them repent, and be prepared for when Hashem will come to judge the land.*
