Michael Lopez has a bit of a Star Wars conundrum, and I would not be Blogistans greatest single contributor to Star Wars fandom* if I didn't weigh in. Michael is concerned about the timeline in between The Empire Strikes Back and Return of the Jedi, specifically in the sense that when Luke leaves Dagobah prematurely in TESB, Yoda and Ben are stunned because he hasn't completed his training, but when Luke returns to Dagobah in ROTJ, Yoda tells him that he needs no further training. What gives?
Michael offers three explanations, and generally I go with a revised version of his first theory. My thoughts hinge on two facets here: first, the fact that we don't know how much time elapses not just between TESB and ROTJ but also within TESB itself; and second, on just what constitutes Luke's "training", or in other words, why Luke went to Dagobah in the first place.
First of all, at least a year seems to have passed between TESB and ROTJ, during which I assume that Luke has been assisting Lando and Chewie in locating Han. I assume this because at the very end of TESB, Luke tells Lando, "I'll meet you at the rendezvous point on Tatooine", which implies to me that as soon as his hand is all ready to go, he's off to help find Han. I can see them spending a year -- it probably takes that long to learn about Jabba's operation, get Lando an "inside job", hatch all the various parts of the plan for Han's rescue, and for Luke to do some work like build his new lightsaber. (This last one is important, because both Vader and the Emperor later see the existence of this new lightsaber as evidence of Luke's abilities, thus implying that not just any old doofus with some tools can build a lightsaber.) Secondly, I've always assumed that the events in TESB had roughly a month to take place, with the bulk of that coming in the scenes between the asteroid field and the arrival of Han and company on Bespin. The film doesn't give too many clues here, but surely what training Luke requires on Dagobah takes more than just a few days, and there is Han's line "It's pretty far, but I think we can make it", in reference to Bespin before they set out on that journey.
Now we come to the more important question: Just what is Yoda teaching Luke, anyway? It's pretty clear that Luke's not getting anything remotely resembling the traditional Jedi training, which starts at an age even younger than the ten years or so that Anakin is when we first meet him in The Phantom Menace, and ends somewhere around Luke's age in TESB (assuming Obi Wan in TPM actually is that same age). He's getting a serious "crash course", a just-the-basics kind of thing. Luke first starts using the Force in A New Hope, on a very rudimentary level; but he proceeds from "The Force? What's that?" to actually drawing upon it in destroying the Death Star pretty quickly. Next time we see him, in TESB, he's able to use the Force to call his lightsaber into his hand, and while we don't see him actually using it until Dagobah, I think it safe to assume he's continued to do so and probably used it in such exploits as in the Battle of Hoth when he brings down a walker by himself.
So Luke spends a month or so on Dagobah learning some stuff, and then runs off to fight Darth Vader. What stuff is he learning? Well, given what little we see of Luke's training activities -- jogging through an intense obstacle course, doing handstands while levitating stuff -- and given the nature of his single most important lesson ("Size matters not"), I tend to believe that Ben doesn't send Luke to Dagobah to learn nuts-and-bolts stuff like lightsaber fighting and whatnot, but to work on the mental aspects of being a Jedi. When Luke decides to leave prematurely, the overriding concern that Yoda and Ben have isn't that Luke's swordplay isn't nearly as good as Vader's and that therefore the Dark Lord will vivisect him, but rather that Luke doesn't have sufficient control of the Force to avoid temptation by the Dark Side. So, in TESB, Luke isn't being "trained" by Yoda in the same sense that Daniel is being "trained" by Mr. Miyagi in The Karate Kid (i.e., that Luke has to be taught Jedi-stuff from scratch with tricks like "R2 unit in, R2 unit out".
What does this mean, then? Well, it implies that Luke's experiences in fighting Vader and then rescuing Han actually constitute a major victory for Yoda and Ben, because Luke doesn't give in. He doesn't join Vader, he isn't tempted by the Dark Side, to the point of choosing certain death (jumping off the gantry into Cloud City's reactor shaft) over joining Vader. Since Luke has made this choice, and since he has further demonstrated Jedi valor by constructing his new lightsaber and using his abilities so impressively against Jabba's entourage, Yoda deems that he is mentally ready to face Vader. This is what Yoda means by "No more training do you require". Of course, Luke doesn't know this, so he goes back to Dagobah, to keep his promise.
A couple of minor things leap to mind in reading Michael's theories:
1. He refers to Yoda being "healthy and spry" in TESB, but I don't think he looked terribly "spry" at all -- he looked old and wrinkled. I didn't find his death in the subsequent film terribly surprising.
2. Michael also seems to reject the "Luke's just that good" theory by saying that Luke isn't that good: "He's no Anakin Skywalker." Well, it seems to me that Luke actually is pretty damned good: with minimal training and experience he destroys the Death Star; he gets his ass kicked by Vader in TESB, but it does take Vader some doing; and in the end of ROTJ, Luke defeats Vader/Anakin in single combat. This last is pretty important. From the first moments of the duel in the Emperor's Throne Room, it's very clear that Luke has progressed immeasurably. He fights with much more speed, he pushes Vader back, he knocks Vader down the stairs. And when he finally gives in to the anger, for that one brief moment, he drives Vader to his knees. Luke, clearly, actually is that good.
I also don't like Michael's third option, that the scene between Luke and Yoda in ROTJ takes place after some time that Luke's been on Dagobah, because I can't believe that Luke wouldn't raise his questions at the first sight of Yoda, and because when Yoda tells Luke he's dying, Luke says, "But I've come back to complete the training". That, too, seems to be something he'd have said earlier, if this isn't the first time they're seeing each other.
(Now, one thing that's often bugged me about that scene in ROTJ is how quickly it's over. I would have liked it better if, say, Ben had met Luke upon his landing and told him, "Yoda is very sick and near death", as opposed to the way the scene plays out as is. It's kind of like Yoda says, "Sick have I become, old and weak...soon I will rest, forever sleep...really soon. Like, you'd better ask me what's on your mind, Luke, because I'm out of here in five minutes. Too bad I just put a pot of rootleaf on the stove -- you'll want to stir that, because it sticks to the pot something awful!"
Anyway, there's my take.
* This title is self-appointed.