"Who wants to eat that apple?"
Translation:Kiu volas manĝi tiun pomon?
I am struggling with this distinction as well. One sentence uses tio for "...want to say that...", but the next uses tiu for "...wants to eat that apple..."
The best I can understand is that tio is used when the "that" is not specified. In my first phrase we don't know what is being said exactly.
In the second phrase tiu is used because the "that" specifically refers to the apple that is mentioned.