Simplify the following text for a Grade 9 native English-speaking person.
Original: Opened in 1860 , it became the preeminent music theater of late 19th century Russia , where many of the stage masterpieces of Tchaikovsky , Mussorgsky , and Rimsky-Korsakov received their premieres .
Simplified: It was opened in 1860 and soon became the most important theater for opera and ballet in Russia . Many great operas and ballets of the 19th century were first performed there , including the greatest stage works by Tchaikovsky , Mussorgsky , and Rimsky-Korsakov .