A free program for Android, by Andrii Kotliarenko.

Music Theory with Piano Tools is a free educational app for learning to play the piano. The app provides a very simple and intuitive user interface, which allows you to quickly and easily navigate through the different topics. Each topic is divided into several sections, and you can choose to follow the topics in order of difficulty.

Every musician should know the basics of music theory, even if you don’t have a lot of experience. This app is a handy tool for those who want to get into the music theory. If you struggle with learning music theory, this app is for you! 

The app contains a list of topics and tools. Topics help you understand the essential concepts of how music works. The tools show you how to use the theory in practice.