The feature on the website to provide feedback per phrase (unnatural phrasing, my answer should be accepted, mangled audio) doesn't exist in the iPhone app.
I feel bad using the app, memorizing the broken English necessary to bypass a question because I feel it's going to skew your analytics on which questions are causing issues.
Feature parity please? :)
I feel the same. I try to practice on my iPhone from time to time and then drop it again because I can't submit a suggested translation or report a mistake. When I practice on the site my frustration over losing hearts is balanced by the pleasant feeling of being useful by submitting alternative translations. I feel like a little active contributor to Duolingo. On the iPhone, I only feel like a passive user of not-so-perfect software.
Giving feedback directly from a sentence in a lesson isn't possible yet on the iPhone app. You can get in touch by clicking on 'feedback' in the profile view. From there, send us an email with all the details. We know it isn't ideal and would like to make it easier. Thanks for letting us know your thoughts!