我简单讲
1,that作指示代词,比如 This is my father.
2,就是你说的关系代词,引导限定性定语从句.在下列情况下多用that:先行词既有人又有物时;先行词有形容词最高级、序数词、不定代词、very,only等修饰时;先行词是不定代词时)
He talked about the teachers and schools that he had visited.
I think it one of the most wonderful films that the film company has ever produced.
She has little information that is useful for our research.
Is there anything that I can do for you?
3,引导宾语从句,主语从句等.
如果有什么不懂的可密我