When can that in predicative clauses be omitted?

This is inevitable in predicative clauses.

In the clause, it can be omitted only in two cases.

First, it can be omitted in the object clause. In addition, when the object clause has more than two sentences, only the sentence in the first clause can be omitted.

Second, in the attributive clause, that refers to the antecedent, which can be omitted when it is used as the object in the guiding clause. For example, this is the book I am looking for. That refers to the book, and as the object of search, it can be omitted at this time.