Complex sentence: MGM had been looking for a suitable actress with a British accent and the ability to ride horses since 1937, and chose Taylor at the recommendation of "White Cliffs" director Clarence Brown, who knew she had the required skills.
Split into simpler sentences: MGM had been looking for a suitable actress with a British accent and the ability to ride horses since 1937. Taylor was cast at the recommendation of director Clarence Brown, who had previously worked with her in "White Cliffs" and knew she had the required skills.