A free app for Android, by Chess King.

Chess Strategy & Tactics Vol 1 1600-2000 ELO is a course for club players based on a textbook by Victor Golenishchev. 

The source material is supplemented with examples of play by lead chess players from the latest major contests and is organized in chess lessons. The course contains 57 themes, including theoretical material and practical exercises. 

The theory is presented in an interactive way, which means you can not only read the text of the lessons but also make moves on the board and work out unclear moves on the board.

Advantages of the program:

You can improve your chess knowledge, learn new tactical tricks and combinations, and consolidate the acquired knowledge into practice.