Complex sentence: Tony Wendice (Ray Milland), a former professional tennis player, is married to wealthy socialite Margot (Grace Kelly), who has had an affair with American crime-fiction writer Mark Halliday (Robert Cummings).
Split into simpler sentences: Tony Wendice (Ray Milland), an ex-professional tennis player, is married to wealthy socialite Margot (Grace Kelly). She had an affair with American crime-fiction writer Mark Halliday (Robert Cummings).