Complex sentence: Since 1980, the majority of Mattapan has been inhabited by foreign born citizens, and until about 1990 many of these foreign born inhabitants became U.S. citizens; after the 90's many of the people that came the United States did not become a U.S. citizen.
Split into simpler sentences: Since 1980, the majority of Mattapan has been inhabited by foreign born citizens, and until about 1990 many of these foreign born inhabitants became U.S. citizens. After the 90's many of the people that came the United States did not become a U.S. citizen.