Complex sentence: The river begins in Angola and serves as the border between Angola and the Democratic Republic of the Congo (DRC), when it flows into the DRC, the Kasai joins the Congo at Kwamouth northeast of Kinshasa.
Split into simpler sentences: The river begins in Angola and then serves as the border between Angola and the Democratic Republic of the Congo (DRC), until it flows into the DRC. The Kasai joins the Congo at Kwamouth northeast of Kinshasa.