Complex sentence: The Kot Jazzmen was founded during the Nazi occupation of Belgium, when a number of Brussels musicians went into hiding and got together in a tiny four-story building on the Rue des Moineaux.
Split into simpler sentences: The Kot Jazzmen was founded during the Nazi occupation of Belgium. At that time, a number of Brussels musicians went into hiding and got together in a tiny four-story building on the Rue des Moineaux.