"Be good", is what we often tell our children (and maybe other people's children too). And I'm sure whatever child we just said that to certainly means to be good. We know exactly what "be good" means. We have the perfect picture in our heads. But does the child know what "be good" means?
To the child, it probably means "don't do anything that makes Mommy (or Daddy, Grandma, Teacher, etc.) mad." The problem is, poor little Jacob/Emily has no idea what that might be. "Hmmm," thinks Jacob/Emily, "I know hitting people gets me in trouble, so as long as I don't hit anyone, I'm being good. Oops! I just got in trouble for throwing something. Okay, no hitting, no throwing. Uh oh! I just got in trouble for running. So no hitting, no throwing, no running. What? Now I'm in trouble for yelling. Maybe I just better sit here by Mommy and not move. That's the only way I won't get in trouble." Poor kid. Now he/she's afraid to try anything. How did this happen?
Children are very literal and concrete. They can't picture what "Be good" looks like in their heads. Let's take going to Storytime as an example. Mom walks in the door, Jacob heads for the blue rug, and Mom says "Be good." Jacob just turned three a couple of months ago and hasn't had much experience with group activities. He really has no idea what's expected of him. Better if, at home or in the car on the way, Mom says, "Jacob, when Storytime starts, I want you to pick a spot on the floor to sit. And while Miss Teresa reads the stories, stay in that spot and listen quietly. You can talk and play with your friends before and after Storytime, but not during." Now Jacob knows exactly what's expected of him. That's so much better than waiting for him to do something wrong and then correcting him.
It's always better to be telling your children what they are supposed to do, than what they aren't. When we get to say "yes" to our kids, and praise their good behavior (because they knew what was expected of them) we end up with happy, positive, confident children.