Complex sentence: Over time, the monarch increasingly acted only on the advice of the Prime Minister; by the nineteenth century, Prime Ministers had the "de facto" power to dissolve Parliament and call a new general election at a time of their choosing.
Split into simpler sentences: Over time, monarchs increasingly acted only on the advice of the Prime Minister. This meant that by the nineteenth century, Prime Ministers had the "de facto" power to dissolve Parliament and call a new election at a time of their choosing.