A magnesium ribbon should be cleaned before burning because when exposed to air, magnesium quickly reacts with oxygen to form a layer of magnesium oxide (MgO) on its surface. This oxide layer is stable, prevents further reaction between magnesium and oxygen, and does not burn easily, which hinders the burning process. Cleaning the ribbon removes this oxide layer, exposing pure magnesium, which burns vigorously and produces a bright flame. Thus, cleaning ensures the magnesium ribbon burns effectively and reacts fully during combustion.