Complex sentence: Osip Mandelstam considered him to be the greatest Russian poet of all time, and he had a profound influence on the Russian Symbolists, especially Innokenty Annensky and Alexander Blok, who declared Fet his "great teacher".
Split into simpler sentences: Osip Mandelstam considered him to be the greatest Russian poet of all time. Fet had a profound influence on the Russian Symbolists, especially Innokenty Annensky and Alexander Blok, who declared Fet his "great teacher".