Complex sentence: It was the main stadium of Seongnam Ilhwa Chunma (currently Seongnam FC) by 2004-now Seongnam FC uses Tancheon Stadium as their home stadium for most of their games.
Split into simpler sentences: It was the main stadium of Seongnam Ilhwa Chunma (currently Seongnam FC) since 2004. Now, Seongnam FC uses Tancheon Stadium as their home stadium for most of their games.