that在宾语从句和在定语从句中的用法是完全不一样的.
在宾语从句中,that作引导词,不充当宾语从句中的任何成分,在口语和非正式文体中常被省略；
eg:I think (that) you're a good student.
而在定语从句中,that作关系代词,既可以指人,也可以指物；在定语从句中,既可以充当主语,也可以充当宾语.充当宾语时候,可以被省略.
eg:I like the book (that) my mother bought me.