You can usually omit "that" when it introduces a noun clause: I think (that) YOU ARE AN EXCELLENT STUDENT. I think (that) PARIS IS VERY BEAUTIFUL. I think( that ) BROCCOLI IS DELICIOUS. In those sentences, "that" is just an introductory word. ***** You may also omit "that" when it is a relative pronoun and the object of the verb: The book THAT you are reading is interesting. ""that" = the book; it is the object of the verb: You are reading that. BUT you may NOT omit "that" if it is a relative pronoun and the subject of the verb: I want the flower THAT is red. "That" = the flower. It is the subject of "is." You cannot omit it, for nobody would say "I want the flower is red."

Not using the word "that" is usually just to shorten sentences. So if we shorten one word or get rid of one, we tend to do it to all the words in the sentence. So, " I think that you are great. " " I think you are great. " " I think you're great. " The last example would be more commonly used if you are going to shorten it and not use "that".