A very good example of diglossia or diglossic relationship between two versions of the same language is the relationship between Egyptian Arabic and modern standard Arabic. Egyptian Arabic is what one would learn at home, while modern standard Arabic is taught at school and the Koran is written in it. If you only know Egyptian Arabic, then you will have to go to school to learn modern standard Arabic, because the two are different enough that you really are acquiring a whole different system, not just cleaning up and formalizing what you've already learned.