Complex sentence: Gribov completed his studies at the university of St. Petersburg (then Leningrad) in 1952, but at first he could find no employment there because of his Jewish background, so he spent two years teaching at an evening institute.
Split into simpler sentences: Gribov completed his studies at the university of St. Petersburg (then Leningrad) in 1952. At first could find no employment there because of his Jewish background, so he spent two years teaching at an evening institute.