Complex sentence: In the 1920s he developed his mature style of Hard-edge paintings, mainly abstract still lifes and landscapes; his use of contemporary subject matter such as cigarette packages, spark plug advertisements and the contemporary American landscape make him a proto-Pop artist.
Split into simpler sentences: In the 1920s he developed his mature style of Hard-edge paintings, mainly abstract still lifes and landscapes. His use of contemporary subject matter such as cigarette packages, spark plug advertisements and the contemporary American landscape make him a proto-Pop artist.