Simplify the following text for a Grade 9 native English-speaking person.
Original: It was in Stockholm where he met his mentor and long-time friend , jazz pianist Alvaro Is Rojas , who taught him piano improvisation , jazz harmony and ear-training for five years , spanning his high school years and academic studies at the Royal Swedish College of Music .
Simplified: Is Rojas taught him piano improvisation and jazz ear-training for five years .