A Zettelkasten Note should always be written using my own words. The note should always represent my current understanding of what I’m trying to learn so that I can both understand it in the future and so I can see how my understanding has developed over time.

When writing about something that isn’t my own thought, I should not simply copy and paste that information; I should always phrase this new information in my own words as well. However, I should still include a reference to the original source to prevent plagiarism and to make it easier for me to revisit the source material in the future or cite it when I decide to write about the topic.

Source: Zettelkasten — How One German Scholar Was So Freakishly Productive