An example of when to use hans/hendes/dens/dets would be something like this "Han hjælper hans/hendes mor" meaning "He helps his/hers mother". In this case it is helping someone elses mothers.
To be honest, many if not most, Danes also struggles with this grammatical principle. If I were you, I wouldnt worry too much about it. Most Danes cant see when it's wrong anyways. Use what feels right to you and if it's wrong, you will get corrected and over time you will get a sense of when to use what. That's unfortunately the best advice I can give :(