Star Wars: The Empire Strikes Back, and Star Wars: Return of the Jedi were separated by a time jump. Luke stops training in Empire Strikes Back to save his friends, but he somehow becomes a real Jedi when he appears in Return of the Jedi. Luke became so powerful in such a short amount of time. How did he accomplish that?
Star Wars comics have always saved the day and revealed lore not told in the films. This is explored in Star Wars #19.
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How did Luke get so strong in Star Wars Return of the Jedi?
After handling the "War of the Bounty Hunters," storyline, Luke realizes he is not prepared to fight Darth Vader. This is an understatement considering he took a beating in Empire Strikes Back. It was about time he got some real training. So Luke goes to the former Jedi locations to see if there's anything he can learn from them.
In the ruins of Am'balaar, he discovers a Holocron. It reacts to his Force ability and shows a grandmaster who had been training Jedi for 800 years. You guessed it—it's Yoda.
It's easy to understand how Luke became so powerful in such a short period of time.
Holocrons have been used since the days of the Old Republic and appear in many games of the series. Despite the comic's end, we can assume that Luke would have learned most of what he needed from this Holocron. Luke is stubborn in Empire Strikes Back (and Return of the Jedi), but he can be patient when needed.
Additionally, it makes sense since Yoda has been training Jedi for 800 years and has been able to pass down his knowledge to a multitude of students. Obi-Wan, Count Dooku, Mace Windu, and many other strong Jedi are among the most notable. In my opinion, Yoda's Holocron is one of the best.