that是形容动词（verb）或名词（noun）的
    形容动词：
    I konw taht you are a student. -> I know you are a student. （可以不需要that）
    I think that you are a good student. -> I think you are a good student.
    形容名词：
    I have a pen that is black. （taht描述这个笔）宾语从句
    The pen that is red is mine. 定语从句