I didnt know the uncle that I inherited the money from为什么只能用that 不能用which

"That" is used to introduce a restrictive clause that provides essential information about the noun that precedes it. In the given sentence, "that" introduces a restrictive clause that provides essential information about the uncle. The sentence implies that the speaker has multiple uncles, but only one of them left them the money. Therefore, the clause is essential to understanding which uncle the speaker is referring to.

On the other hand, "which" is used to introduce a non-restrictive clause that provides additional information about the preceding noun. Non-restrictive clauses are not essential to the meaning of the sentence and can be removed without altering the meaning of the sentence. In this case, using "which" would imply that the speaker only has one uncle, and the fact that they inherited money from him is just additional information. However, this contradicts the fact that the sentence implies the speaker has multiple uncles. Therefore, "which" cannot be used in this sentence