This is the 306th most frequent Croatian word.
napisao
"Wrote."
Here, 'napisao' is used in the past tense to indicate that Ivan completed the action of writing a letter in the past.
Ivan je napisao pismo svom prijatelju.
Ivan wrote a letter to his friend.
In this question, 'napisao' is used to inquire about the author of a book, emphasizing the completed action in the past.
Tko je napisao ovu knjigu?
Who wrote this book?
In this sentence, 'napisao bih' is used to express a conditional action that would occur under specific conditions in the present or future.