Complex sentence: The Australians were then subjected to heavy artillery and mortar fire at 20:00, before waves of Chinese attacked the Australians at 20:50, penetrating between C Company on Hill 121 and the nearest US Marines on Hill 111, before being turned back amidst the driving rain.
Split into simpler sentences: The Australians were then subjected to heavy artillery and mortar fire at 20:00. Waves of Chinese attacked the Australians at 20:50, penetrating between C Company on Hill 121 and the nearest US Marines on Hill 111, before being turned back amidst the driving rain.