Translation:While he is writing a letter, he is reading a book.
April 8, 2013
Only "quand" is versatile and can be used to ask a question. The other two are used as conjunctions only and "tandis" implies a duration (while)
- quand écrit-il ? (when does he write?)
- il écrit quand/lorsqu'il a le temps (when he has time)
- il écrit quand/lorsque/tandis que... je lis (... while I read)