Complex sentence: In the pre-independence era, he was a leading member of the Communist Party of India Upon partition, he moved with his family to newly-created Pakistan and in 1948, he became a founding member of the Communist Party of Pakistan.
Split into simpler sentences: In the pre-independence era, he was a leading member of the Communist Party of India. Upon independence and partition, he moved with his family to newly-created Pakistan and became a founding member of the Communist Party of Pakistan.