Cambridge English Grammar in Use by Raymond Murphy. The first section on Past and present is free.(If you want to do other tenses in this app, you need to buy it.) Do the 5 exercises in Unit 6 (Past Continuous), then write a paragraph of 8 lines to say:

• how useful this was for learning English and why
• how easy the app was to use 
• how interesting the app was 
• if you would use the app again

The 5 exercises in Unit 6 is easy for me, so I don't like to use it again. I would like to learn something hard a bit to challenge myself and train my brain to think. However, this Unit  is useful for the people learning English for the beginner because it has lessons to explain and has also easy exercises to do.