Complex sentence: The Tay Son kept pursuiting lord Nguyen Phuc Thuan and forced him to call for reinforcement from Nguyen garrison in Saigon but the Nguyen governor of Saigon couldn't move his troop in time which gave Đỗ Thanh Nhơn the chance to aid the Nguyen lord.
Split into simpler sentences: The Tay Son kept pursuiting lord Nguyen Phuc Thuan and forced him to call for reinforcement from Nguyen garrison in Saigon but the Nguyen governor of Saigon couldn't move his troop in time. This situation gave Đỗ Thanh Nhơn the chance to aid the Nguyen lord.