"Ningún hombre sabe cuán malo es hasta que ha intentado con mucha fuerza ser bueno."

Simplified Meaning:

When people try their best to be good, they often discover that it's really difficult. This shows them what they're really like on the inside. For example, imagine someone who's trying to be more patient. They might find out that they get easily angry or frustrated, more than they realized before. It's only when they push themselves to be calm and kind that they see how quickly they can lose their temper. In everyday life, when someone tries to be honest all the time, they might notice all the little lies they used to tell. Or, if someone tries to be really generous, they might realize they often think about themselves first. This doesn't mean they're bad people; it just shows them the areas where they can grow. By seeing their flaws, people can work on becoming better. It's like when you clean a room and find all the dust hiding in corners you didn't see before. Knowing where you're not perfect helps you improve and become the kind of person you want to be.