Complex sentence: Trepca once accounted for 70 percent of Kosovo's gross domestic product, but since the war ended in 1999, the partition of Mitrovica between Serbs and ethnic Albanians has kept most of Trepca's facilities closed.
Split into simpler sentences: Trepca once accounted for 70 percent of Kosovo's gross domestic product (GDP). But since the war ended in 1999, the partition of Mitrovica between Serbs and ethnic Albanians has kept most of Trepca's facilities closed.