Simplify the following text for a Grade 7 native English-speaking person.
Original: The conductor 's long and fruitful association with the Royal Opera House , Covent Garden , began in 1952 with his appointment as an assistant to Rafael KubelÃ k. His first job was prompting Maria Callas . He remained a company member for 17 years , returning annually thereafter as a guest conductor before assuming the post of Associate Music Director in 1991 .
Simplified: In 1952 he started his work in the Royal Opera House , Covent Garden where he was assistant to Rafael KubelÃ k. He worked for the ROH for 17 years , becoming Associate Music Director in 1991 .