Complex sentence: Short comments in various episodes act as hints, but the war was not thoroughly talked about until the 2007 series finale; the two-part special "The End of Time" (2009) provided further information.
Split into simpler sentences: Short comments in various episodes act as hints, but the war was not thoroughly talked about until the 2007 series finale. The two-part special "The End of Time" (2009) provided further information.