Complex sentence: Instrumental in this decision was Pyotr Pletnyov, the University professor who, acting as a mentor for the young man, showed the first poems of his protégé to such literary giants as Vasily Zhukovsky and Nikolai Gogol.
Split into simpler sentences: Instrumental in this decision was Pyotr Pletnyov, the University professor who was acting as a mentor for the young man. Pletnyov showed the first poems of his protégé to such literary giants as Vasily Zhukovsky and Nikolai Gogol.