I noticed that it took some time to process each answer. If I had made this, I would probably have loaded the answers and image URLs for each lesson only once, in a single JS file (preferably a static file). Then you would not have to connect to the web server for each answer.
And try to keep the user's progress in a cookie if it's possible. Make the login optional if a user wants to save their progress permanently.
Yes, I probably should preload all the answers for the questions. However, we store the mistakes the user does, so the server has to be contacted each time anyways. And sending back to the server will be useful for later when we mix in sentences and all that.
And try to keep the user's progress in a cookie if it's possible. Make the login optional if a user wants to save their progress permanently.