"Do you believe that?"
Translation:Ĉu vi kredas tion?
"tion" = this. It's a pronoun and stands alone. It could refer to any kind of thing.
"tiun" = this one. It's a demonstrative adjective and usually stands before a noun; if you leave it out, then in English you have to add the dummy noun "one" (like "the red one, the blue one, this one"). The context would have to make clear what kind of noun is missing.
Or "tiun" = this person. The -u series can also refer to people (kiu = who? as well as which (one)?"
tiun usually stands before a noun; if you leave that noun out (and just have 'tiun' by itself), then in English you can't leave the noun out -- just as with other adjectives, you can't just say "I want the red" or "The pink is cheaper" but it has to be "I want the red one" and "The pink one is cheaper" -- but add the dummy noun "one".
So "Mi volas tion" = I want that, but "Mi volas tiun" = I want that one.
Oh, I see; thank you!
(Except that one does just say, when the objects are present, for example if you are offering a bin of lollipops or toy cars to a child: "Which do you want, the red, the green, or the blue?" "I want the red." I suppose the physical presence stands in for the noun.)
Edit: this would be the native New England version of English.