Complex sentence: After his Olympic success, Thompson won his third Commonwealth title in 1986 but after that he never quite recaptured the first decathlon defeat for nine years when he finished ninth in the World Championships, and at his third Olympics in Seoul in 1988 he finished fourth.
Split into simpler sentences: After his Olympic success, Thompson won his third Commonwealth title in 1986 but after that he never quite recaptured the superlative form of earlier years. In 1987 he suffered his first decathlon defeat for nine years when he finished ninth in the World Championships, and at his third Olympics in Seoul in 1988 he finished fourth.