Complex sentence: Before his campaigns of 1938 and 1939, Muñoz Marín met while legally still married Inés Mendoza, his mistress a teacher who was fired for complaining about the prohibition against classes in Spanish.
Split into simpler sentences: Before his campaigns of 1938 and 1939, while he was still legally married, Muñoz Marín met Inés Mendoza. A teacher, she became his mistress and was fired for complaining about the prohibition against classes in Spanish.