he suddenly fell ill made us surprised.如果不加THAT也不会引起歧义啊

He suddenly fell ill,which made us surprised.
定语从句,which指的是这件事,作为从句的主语,不能省.
His suddenly falling ill made us surprised.
非谓语,你的原句有点问题,不是felling而是falling,还有一楼说的是错的!不用makes而是用made.至于为什么是"fallen"是因为他生病是主动的.