How we cite our quotes: All quotations are from Star Wars: The Empire Strikes Back.
Quote #4
LUKE: But how am I to know the good side from the bad?
YODA: You will know. When you are calm. At peace. Passive. A Jedi uses the Force for knowledge and defense, never for attack.
According to Yoda, a Jedi must always be in control of his or her emotions in order to keep from straying to the dark side. As it happens, Luke has a hard time with this. It's not like he doesn't have any reason to be angry—Vader massacred his entire family in the previous film, after all—but it's still a flashing warning sign for the future.
Quote #5
YODA: That place is strong with the dark side of the Force. A domain of evil it is. In you must go.
LUKE: What's in there?
YODA: Only what you take with you.
Luke faces his greatest challenge yet in the cave and... fails. The key to this passage is Yoda's claim that the only thing that's in the cave is "what you take with you," which indicates that the shocking scene that follows (which we'll discuss in detail shortly) is some sort of reflection of Luke's inner turmoil.