"Marcus writes a book."
Translation:Marcus librum scribit.
Liber when the book is doing the action or simply describing the book:
- Liber est meus -> "The book is mine." - Describing the book by linking an adjective to it with est.
- Liber caseum olet -> "The books smells like cheese." - The book is doing the action of 'giving off a scent'.
Librum is used when an action is being done to the book:
- Librum habeo -> "I have the book."
- The sentence here, Marcus is doing an action (writing) to a book.
Marce is used when you are directly addressing him, talking to him, yes. Compare Marce, librum scribis ("Marcus, you are writing a book.") to the Duolingo sentence. My sentence is a statement that is said directly to Marcus, while the Duolingo sentence is about Marcus, but not directed at him.