In a physical book, I would circle or underline something and write in the margins, e.g. if I encountered the word “voudrais” and did not know it, I would mark it, look it up, and then write in the margins “ conditional tense of ‘vouloir’”. So, it would be helpful to be able to e.g. highlight a word that I would like to revisit, or make a note/comment on a word or phrase, e.g. translating it into my own language or writing up use cases.