"While you read your book, I write a letter."
Translation:Pendant que tu lis ton livre, j'écris une lettre.
Thanks! Only after I posted my question did I find the link below, which expands on your answer. Here it is for anyone else who's confused: http://french.about.com/od/grammar/a/quand-vs-lorsque.htm
yes, you switched the main clause with the conjunctive clause. in terms of meaning, it does not change much, since the two actions are happening in parallel, but remember that Duo's software cannot decipher as right a sentence where you change the word order with no good reason to do so.
The correct translation is: "Pendant que vous lisez votre livre, j'écris une lettre", so the use of votre livre is correct. There must have been some other mistake in your translation.
Informally, you could also write: "Pendant que tu lis ton livre, j'écris une lettre".
Votre and vous must be paired, while tu and ton are paired. :-)
When a short word, which ends with a vowel, is followed by a word, which begins with a vowel, the ending vowel is dropped from the first word and an apostrophe joins the two words: