Simplify the following text for a Grade 9 native English-speaking person.
Original: Claudio Abbado became principal conductor after Karajan , expanding the orchestra 's repertoire beyond the core classical and romantic works into more modern 20th-century works .
Simplified: After Karajan , Claudio Abbado became principal conductor . He helped the orchestra to learn new works , not just the well known classical and romantic works but also the more modern 20th century works .