Complex sentence: Instrumental in this was P.A.Pletnyiov, the University professor who greatly encouraged the young man, was fulfilling the role of his mentor and showed his first poems to such literary giants as Vasily Zhukovsky and Nikolai Gogol.
Split into simpler sentences: Instrumental in this decision was P.A.Pletnyiov, the University professor who greatly encouraged the young man. Fulfilling the role of a mentor, Pletnyov showed the first poems of his protégé to such literary giants as Vasily Zhukovsky and Nikolai Gogol.