Complex sentence: He became Mayor of Amsterdam, serving as Mayor from 16 June 1983 until resignation on 18 January 1994 to become again Minister of the Interior serving from 18 January 1994 until 27 May 1994, when he resigned following the "IRT-affair".
Split into simpler sentences: He became Mayor of Amsterdam, serving as Mayor from 16 June 1983 until resignation on 18 January 1994. In January 1994, van Thijn became Minister of the Interior again serving from 18 January 1994 until 27 May 1994, when he resigned following the "IRT-affair".