Complex sentence: Efrain moved to the U.S. from Mexico as an illegal immigrant, where he met American-born Isabel, they started a relationship and later married, which allowed Efrain to attain U.S. citizenship.
Split into simpler sentences: Efrain moved to the U.S. from Mexico as an illegal immigrant, where he met American-born Isabel. They started a relationship and later married, which allowed Efrain to attain U.S. citizenship.