Complex sentence: Mickiewicz enrolled in the Yale School of Music in 1953, earning a B.A. in Music at Yale University in 1957, After that he studied Slavic Languages and Literature, receiving an M.A. and then a Ph.D. from Yale.
Split into simpler sentences: Mickiewicz enrolled in the Yale School of Music in 1953, earning a B.A. in Music at Yale University in 1957. He then entered the graduate program in Slavic Languages and Literatures at Yale, receiving an M.A. and then a Ph.D. from Yale.