Complex sentence: A Tortomon was seen rampaging through the city, but when Masaru Digivolved Agumon into GeoGreymon, he was quickly defeated and reverted into his Digitama form.
Split into simpler sentences: A Tortomon was seen rampaging through the city. But when Masaru Digivolved Agumon into GeoGreymon, he was quickly defeated and reverted into his Digitama form.