Complex sentence: Pushkin was analysed by Nikolai Gogol, his successor and pupil, and the great Russian critic Vissarion Belinsky who has also produced the fullest and deepest critical study of Pushkin's work, which still retains much of its relevance.
Split into simpler sentences: Pushkin was analysed by Nikolai Gogol, his successor and pupil, and the great Russian critic Vissarion Belinsky. The last mentioned also produced the fullest and deepest critical study of Pushkin's work, which still retains much of its relevance.