Simplify the following text for a Grade 11 native English-speaking person.
Original: During Mengelbergâ s time the â first conductorsâ included Karl Muck ( 1921-1925 ) , Pierre Monteux ( 1924-1934 ) , Bruno Walter ( 1934-1939 ) , and Eugen Jochum ( 1941-1943 ) , and the second conductors included the composer Cornelis Dopper , Evert Cornelis and Eduard van Beinum .
Simplified: During Mengelberg 's time as chief conductor , several of these first conductors included Karl Muck ( 1921 â `` 1925 ) , Pierre Monteux ( 1924 â `` 1934 ) , Bruno Walter ( 1934 â `` 1939 ) , and Eugen Jochum ( 1941 â `` 1943 ) . Musicians who served as `` second conductor '' included the composer Cornelis Dopper , Evert Cornelis and Eduard van Beinum .