Simplify the following text for a Grade 11 native English-speaking person.
Original: He was the principal conductor of the Royal Philharmonic Orchestra from 1987 to 1994 , and was principal conductor of the Czech Philharmonic from 1998 to 2003 .
Simplified: He was principal conductor of the Royal Philharmonic Orchestra from 1987 to 1994 , and of the Czech Philharmonic from 1998 to 2003 .