Complex sentence: He taught painting for several decades in California commencing in the early 1950s when he taught in Oakland and then, after serving as a visting artist at several universities, he became a Professor of Studio Art at Stanford University.
Split into simpler sentences: He taught painting for several decades in California beginning in the early 1950s when he taught in Oakland. Then, after serving as a visiting artist at several universities, he became a Professor of Studio Art at Stanford University.