"That" can be used in several different ways that can be confusing. 1. The relative pronoun "that" will serve as a subject or direct object in its clause. a. She is the one [that I love]. (I love that (one); "that" = direct object of "love")(as a direct object, this "that" may be omitted usually) b. She is the one [that will marry you someday]. ("that" = subject of "will marry")(as subject, this "that" can NOT be omitted) 2. The noun-clause-flag "that" has NO meaning and NO function in the sentence other than to visually/audibly alert the reader/listener that all the words that follow will be a noun clause that will be treated as a single noun. I think [that he is a good teacher] and [that he is a good student, too]. Is it correct? Yes. I think A and B. A = he is a good teacher. B = he is a good student. These noun clauses are direct objects. The "that" in A and B simply says, "All the words that follow will be a noun clause, so be listening for the beginning and end of it." Sometimes this "that" can be omitted without losing clarity, but sometimes it will be needed for clarity. Note that if a noun clause is the subject of a sentence, this "that" will be hanging out in the front of the noun clause; in this case, it is required. [That he is too tired to continue] is obvious to me. (noun clause used as a subject. The leading "that" tells the listener that a noun clause will follow and will be the subject of the rest of the sentence.)

We can almost always use the clause with or without the word "that". I think (that) he is a good teacher, and (that) he is a good student, too. I think he is a good teacher, and he is a good student, too. Both are correct. But if we don't have to use "that", why would we? Using the word "that" emphasises (that) the connection is strong, and helps sometimes to prevent misreading. A. "I am sorry that you can’t come to the party." (I am sorry because you can't come to the party) B. "I'm sorry you can't come to the party." (Same as A) C. "I'm sorry, you can't come to the party." (I don't want you to come to the party.)

I'm really struggling to understand this question. Can you try rewording your question?