Question

that从句是什么？

Answer 1

that引导定语从句时，它前面的先行词既可指人也可指物，that在定语从句中可作主语、宾语或表语，作宾语时that可省掉，作主语或者表语时不能省

Answer 2

That从句其实就是that引导的从句，that可以引导宾语从句，也可以引导定语从句。

Answer 3

要根据具体的语境和句子意思，可以引导宾语从句，也可以引导定语从句。
如
He says that he will be Free tomorrow.引导宾语从句
他说他明天会有空。

Can you see the book that I bought yesterday? 引导定语从句
你看见我昨天买的那本书吗？

Answer 4

没有上下文吗？
that 可以是宾语从句，表语从句，定语从句等
如
He says that he is a student. 宾语从句

Can you see the book that I bought yesterday.(定语从句)