Complex sentence: After Sheridan's "return from the dead", in the fourth season he displays an increasing impatience with accomplishing his goals; however, this is caused by the fact that he knows he has only twenty more years to live.
Split into simpler sentences: After Sheridan's "return from the dead" in the fourth season, he displays an increasing impatience with accomplishing his goals. This is caused, however, by the fact that he knows he has only twenty more years to live.