Complex sentence: He subsequently obtained a postgraduate degree in public law and held various positions in the civil service until becoming Clerk of the Privy Council of Canada and Secretary to Cabinet under Pierre Trudeau from 1975 to 1979 and again from 1980 to 1982.
Split into simpler sentences: He subsequently obtained a postgraduate degree in public law and held various positions in the civil service. He became Clerk of the Privy Council of Canada and Secretary to Cabinet under Prime Minister Pierre Trudeau from 1975 to 1979, and again from 1980 to 1982.