Complex sentence: On the way back, Nauls cuts MacReady loose from the tow line-he believed that MacReady had been assimilated when he found a torn shirt with MacReady's name on it.
Split into simpler sentences: On the way back, Nauls cuts MacReady loose from the tow line. He assumes that MacReady has been assimilated when he finds a torn shirt with MacReady's name on it.