I can't remember my username, how do I log in?

If you forgot your username, you can sign in with your email address.   

Once you are able to successfully log into Duolingo app, you will be able to view your username by tapping the Profile tab, then Settings/Gear icon (top-right). 

If you are logging in via the web version, you can find your username here: https://www.duolingo.com/settings