Complex sentence: At the time, Zhong Hui was practising filial mourning because his mother had recently died, but he immediately interrupted his mourning period and went to warn Sima Zhao when he foresaw that Zhuge Dan would disobey the order.
Split into simpler sentences: At the time, Zhong Hui was practising filial mourning because his mother recently died. However, he immediately stopped mourning and went to warn Sima Zhao when he foresaw that Zhuge Dan would disobey the order.