Complex sentence: Nabokov was made secretary to the Provisional Government; however, he was forced to leave St. Petersburg in December 1917 after the Provisional Government was overthrown by the Bolshevik revolution.
Split into simpler sentences: Nabokov was made secretary to the Provisional Government. However he was forced to leave St. Petersburg in December 1917 after the Provisional Government was overthrown by the Bolshevik revolution.