Complex sentence: The Netherlands has been a constitutional monarchy since 1815 and a parliamentary democracy since 1848 before that it had been a republic from 1581 to 1806 and a kingdom between 1806 and 1810 (it was part of France between 1810 and 1813).
Split into simpler sentences: The Netherlands has been a constitutional monarchy since 1815 and a parliamentary democracy since 1848. Previously, it was a republic from 1581 to 1806, and a kingdom between 1806 and 1810 (it was part of France between 1810 and 1813).