Complex sentence: Ivan Pavlov married Seraphima Vasilievna Karchevskaya on 1 May 1881, whom he had met in 1878 or 1879 when she went to St. Petersburg to study at the Pedagogical Institute.
Split into simpler sentences: Ivan Pavlov married Seraphima Vasilievna Karchevskaya on 1 May 1881. They met in 1878 or 1879 when Seraphima went to St. Petersburg to study at the Pedagogical Institute.