Complex sentence: He and his wife were founder members of the CoBrA movement in 1948, remaining with the group until it dissolved in 1951, producing free-form, spontaneous images in srong, fantastic colours.
Split into simpler sentences: He and his wife were founder members of the CoBrA movement in 1948. They remaining with the group until it dissolved in 1951, producing free-form, spontaneous images in strong, fantastic colours.