Simplify the following text for a Grade 11 native English-speaking person.
Original: On 21 April 1789 , he married Archduchess Maria Teresa of Austria-Este , daughter of Ferdinand , Duke of Modena ( who was the son of Francis I , Holy Roman Emperor ) .
Simplified: On 21 April 1789 , he married Archduchess Maria Teresa of Austria-Este , daughter of Ferdinand , Duke of Modena .