Complex sentence: However, Sigismund could seize the government of Hungary only when the queen and her mother were captured by some rebellious barons (25 July 1386) and appointed Stibor to his Master of the Court.
Split into simpler sentences: However, Sigismund could only seize the government of Hungary once the queen and her mother were captured by some rebellious barons (25 July 1386). He then appointed Stibor to his Master of the Court.