1.I have heard someting. something作宾语,that可省
2.his friend gave him the birthday present. presents做宾语补足语,that/which 可省
3.先行词是人且是主语,用who.如,the man who is waiting for you
先行词是人,且作宾语,用whom.如,the girl whom the police are looking for
先行词是指代谁的,用whose.如,the student whose mother is ill
找点例句,单单讲理论解是不容易理解的