Do I need the internet to use Duolingo?

Yes and no. If you are accessing Duolingo via the web, you need to be connected to the internet.  

If you have a mobile device, we have offline mode for our Android and iOS apps, which allows you to access a handful of lessons without internet or wifi connectivity. However, not all of your lessons are available to you when in offline mode. You’ll need to connect to the Internet for those lessons to be recorded and count towards your daily progress.