When should I punish a behavior?

Never. I know that sounds silly because puppies certainly can be what we consider naughty—getting into things, chewing things, eating things they're not supposed to, but what we need to teach them is what do you want them to do? What sort of behaviors do you want in your environment? Because if you just tell your puppy, "I don't want you to chew on the couch," that doesn't tell them anything. They don't understand that. So we need to say, "Hey, chew on this. This is a good idea." Or, "I don't want you to pee in my living room; let’s pee outside and make a party out of it when you do it right." We want to reinforce those behaviors that we like and that we need to keep your harmonious household or keep your pet safe. Punishment doesn't work. Punishment tells them in the moment that something terrifying happens when they pee on the carpet, but it doesn't teach them what you want them to do. And all it does is damage your relationship with your puppy.