tio

Tio names a thing or a whole idea without identifying it: "that", as in "that is good". It takes -n when it is the object, giving tion, but it never takes the plural -j, because it is already an unnamed lump rather than a countable item. The moment a noun follows, switch to tiu: tio estas bona, but tiu lingvo estas bona. Its question-word partner is kio, and adding ĉi brings it close: ĉi tio is "this".