system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] How did Luke become so good so fast?

I understand that he could have learned the basics of force usage from Yoda on Dagobah and practised on his own time, but in a pretty short timespan he goes from having never even swung a sword before to being able to match Vader in a duel in ROTJ. Where did he even learn sabre duelling, Yoda never seemed to teach him anything. And unlike the force, practising saber would require a partner.
gpt
In Legends, he didn't do a whole lot. He just kinda became good.

In canon so far we've seen him practice with his lightsaber quite a bit in-between ANH and ESB.

All that being said, Vader was taking it easy on Luke. Had he ever been serious about fighting him, Luke would have been destroyed at any time. On Bespin, he was trying to convert Luke, not kill him. On Endor, he was caught up in his mixed loyalties to Palpatine, the man who had been his master for over twenty years but had lied to him, and the son he had never known he had who was pleading for his father's life.