Complex sentence: Mary Anne Clarke (born Mary Anne Thompson 3 April 1776--21 June 1852) was the mistress of Frederick, Duke of York and became the Duke's mistress in 1803, while he was Commander-in-Chief of the army.
Split into simpler sentences: Mary Anne Clarke (born Mary Anne Thompson 3 April 1776--21 June 1852) was the mistress of Frederick, Duke of York. Their relationship began in 1803, while he was Commander-in-Chief of the army.