Complex sentence: On May of 631, Ichan Chilsuk (이찬 칠숙) and Achan Seokpum (아찬 석품) planned a rebellion, however, their plan was discovered and suppressed early on.
Split into simpler sentences: On May of 631, Ichan Chilsuk (이찬 칠숙) and Achan Seokpum (아찬 석품) planned a rebellion. But their plan was discovered and suppressed early on.