Complex sentence: In 1998 one out of the 6 submarines was captured by the South Koreans and so in July 2007,4 were given to Iran to repay some of North Korea's debts to Iran, the last one was retired but kept by the North Koreans.
Split into simpler sentences: In 1998 one out of the 6 submarines was captured by the South Koreans. In July 2007,4 were given to Iran to repay some of North Korea's debts to Iran, the last one was retired but kept by the North Koreans.