I wonder if there is any difference grammatically between "that(1)" and "that(2)".
1. I can't believe that he's only 17.
2. What was that you were trying to say?
I think "3" is correct, I wonder if "4" is correct.
3. I can't believe that he's only 17.= I can't believe he's only 17.
4. What was that you were trying to say? = What was you were trying to say?