So what is a good reason to trade bishop for knight? Most of the time, you should either be
1) winning a pawn,
2) opening up a king (in a situation where it's realistic to attack) or
3) creating a real pawn weakness that you can attack.
It's also ok to trade if your bishop is bad and you have no good retreat square for it.
Of course, there are some situations that don't fall into these categories, but I make it clear that students are responsible to me for having a good reason.
I explain to the class that I'm going to give them 8 positions and they have to tell me if white should trade the bishop for the knight, or not. Notice that in most (5/8) positions, the answer is simply no, you should not take the knight.
Here they are: (or here if you'd prefer a cbv file)
|4. yes, since white can follow up by taking on e5|
|8. No. What white would actually like to do here is play Bg5-h4-g3 and try to trade off black's excellent dark squared bishop.|