Like when Atticus said that you only know someone if you walk around in their skin, and Scout is on the Radley Place and stands in Boo's perspective
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according to what I have seen in the movie I feel the first lesson they learnt is to never kill a mockingbird. Cos they are innocent and only sing for us to enjoy. The second is not to have prejudice on Boo Radley. They always judge before actually understanding him like you said. Even though he is really innocent. I'm not sure about third this is how far I have watched in the movie. Hope it helps you.
To kill a mockingbird/ Putlocker.com