Originally posted by AncientPower
ROTJ Luke has to learn the forms after he invented three new styles in Jedi Academy.What are you on about?
In his final assault on Vader, Luke Skywalker mirrors Vader's own Form V technique and responds with a furious demonstration of Form V's raw strength. An observing Jedi Master would be astonished at such instantaneous learning in battle.
--Insider #62: Fightsaber
[Vader's] style borrowed elements from all techniques of combat, even from the highest, most dangerous levels, and his moves were crisp and unpredictable.
--Dark Lord: The Rise of Darth Vader
[Galen] knew the intimacies of [Vader's] refined version of Djem So, a fighting style that incorporated elements of Ataru, Soresu and Makashi
--The Force Unleashed
Note that Luke wasn't just using a standard variant of Form V, but he was specifically mirroring Vader's application of it, has elements of the other forms in it, so Luke knows them as well.
Originally posted by Tondemonai
Or it's simply because of his rage amp and raw potential (which Vader notes is directly responsible for much of his prowess in saber combat in ESB)
That he mirrors Vader's form? Well, obviously it's due to his potential that he could replicate it after seeing it only a few times. Not that it changes the point in anyway, though.