Yoda:
Told you, did he?
Luke:
Yes
Yoda:
Unexpected this is, and unfortunate
Why did Yoda think Darth Vader would not tell Luke? If they thought it was a secret to Vader, they did a bad job hiding it since Anakin and Luke have the same last name. So, they knew he would know and certainly it would be likely that he would tell Luke the truth.
Why would Yoda(and perhaps Obi-Wan) think it is unexpected that Vader would tell Luke?
Yoda does not elaborate what exactly is “unexpected”, and its just as plausible that its not the fact that Vader told him, but that Luke seemed just fine with it and Vader/Anakin.
And frankly, hiding him on Darth’s home planet and his only living (sorta) relatives, with Anakin’s old sensei living down the road as Ben Kenobi (!) shows that deep thinking not Yoda’s forte it was.
Similar to Obi-Wan*, perhaps Yoda thought what remained of Anakin under that cloak & armor had ceased to exist.
*“He betrayed and murdered your father.”
It could be as simple that it was unexpected that Luke and Vader would meet, have a conversation about the family tree, and then part ways. A conversation like Ren and Han was much more likely.
As for Luke living on Tatooine, that actually seems like the best place. Of all the gin joints in all the planets in all the galaxy to search, Vader would never look in the “obvious” place, so that’s the best place to stash him. It was no coincidence that Obi Wan was there, He was probably running force-interference or something.
And for Yoda’s “unfortunate” comment, it isn’t clear even to him which way Luke is going to go (always in motion is the future), so Vader telling Luke might send Luke to the dark side forever. My take: they wanted Luke more trained in the bright side (what is the opposite of the dark side? The white side? The sunny side? The shiny side?) before he would confront Vader, but Vader pushed upon the timetable.
Sometimes, it’s better to hide under the porch than to hide in the woods.
Or perhaps, Lucas didn’t think of it when he wrote the first movie.
Yeah, I think what they relied on was the Emperor and Vader believing that Padme and her child had both died and therefore, having no reason to look in the first place.
Vader clearly knew sometime before ESB at least.
Plus there was all that SAND. Anakin made no bones bout the fact that he really hated sand—it gets everywhere.
Remember that Yoda didn’t want Luke to go confront Vader. So in that sense it was unexpected; it wasn’t part of the plan, such as it was.
Eh, not really. Even if you assume that Vader only had, say, a 1% chance of looking in the obvious place, we’re talking a galaxy of a million planets. Anywhere else at all would have had a much better chance of Luke staying hidden.
Seriously.
All this retconning is overkill–there are over 46K hits for “Unexpected this is, and unfortunate”! (Not all of them may be trying to make sense of the dialogue, of course.)
Presumably Kasdan and Lucas wanted to give Yoda some dialogue that would introduce tension, so they gave him something to say that would make viewers anxious. Unexpected! Oh, noes! Things may be about to go very wrong! etc.
I think we all know all this. It’s just that some find the Doylist perspective less fun and interesting. 
something else too the skywalker family tree was something that was thought up pretty much on the fly because han didn’t die as ford wanted and important studio people liked the han/Leia romance more than Leia/luke
I agree. Yoda never expected to that he’d face a Luke who had learned from Vader about his (Luke’s) parentage. He expected that Luke would kill Vader before the subject came up, or that Vader would kill Luke, or that they’d battle and Luke would escape before conversation could occur. Yoda might even have expected that Luke would learn that Vader was his father and turn to the Dark Side - but then Luke wouldn’t be coming back to Yoda with questions. The unexpected situation of Luke and Vader meeting, talking, and then Vader not killing Luke and Luke still managing to escape was pretty unexpected - for good reason.
Ya’ll are reaching. “Hiding” him with Skywalker relatives on the same world where Vader grew up, with Obi-Wan just down the road still under the same freakin’ last name by which Vader knew him when both were Jedi knights, was stoopid with two O’s. There’s no way around it. There had to have be plenty of dependable families (like the Organas) with whom baby Luke could have been safely left, discreetly guarded by Obi-Wan, both under assumed names.
The revelation that Darth Vader was Anakin Skywalker, Luke’s father, came as a big surprise to those of use who watched The Empire Strikes Back in theaters when it originally came out. Nowadays, it seems that “Luke, I am your father” is the one thing that everyone does know going into the Star Wars movies for the first time.
Oh, I agree with that. It’s clear that Lucas had no idea that Luke was Vader’s son when he filmed “Star Wars” (no number needed). It’s a coincidence in-movie that the son of someone killed by Vader is nearby when Leia sends a message to Kenobi (Vader kills a lot of people) and a coincidence external to the movie that “Vader” happens to mean “father” in Dutch.
Hold on there; Yoda only ever said that Luke must confront Vader. He never told Luke he had to kill him.
And yet, Vader never did go searching for him, so it’s a moot point. As far as we can tell from the movies alone, Vader had no reason to know his son was alive until Palpatine told him so in ESB.