When someone asks a question, is the reply, "That I don't know" correct? Or is the simpler "I don't know" a more correct or proper response?
I disagree with tchrist on "That I don't know" being unusual. It is very common but has a different connotation. Saying "That I don't know" would be said when you had already answered a question or two, but did not know the answer to the current question. As in
What time are we leaving?
Is Barry going with us?
That I don't know.