Yes, the app is free, but it is difficult to learn on the free version. You can only make up to five mistakes a day. That means you have very few opportunities to learn from your own mistakes and grow. Making mistakes is a way to learn, and you have to feel comfortable making mistakes, making mistakes as many times as you need to. Sure, you can buy more "lives" to keep "learning", just like a war game. In the end, Duo has very little to do with growth and learning.