Tiu vs. Tio
How do you know when to use "tiu" or "tio"? "Kiu" means who, while "kio" means what, if I understood everything correctly, but then I see sentences like "Tiu objekto estas masxino." An object isn't a who, it's a what. Can somebody explain this please?
1. In combination with a noun, use tiu (never tio), like you'd use an adjective.
-- Tiu libro estas tro multekosta. - That book is too expensive.
2. Use tio by itself.
-- Tio estas mia plej favorata libro. - That is my favourite book.
3. If it's clear from context that you are talking about a particular range of objects, and you wish to pick out one particular specimen from that range, you may use tiu by itself, with the referent strongly implied. This is equivalent to the English that one. For example:
-- Jen la libroj, kiujn mi vendas. - Bone. Kiom kostas tiu [libro]? Here are the books I'm selling. - Great. How much does that one cost?
4. Standalone tiu is the demonstrative equivalent of the terms: "everybody" (ĉiu), "somebody" (iu), "nobody" (neniu). Think of it as meaning "thatbody"! Most anglophones don't have problems with neniu and iu (nobody, somebody). So remember that tiu follows the same pattern - it would be grammatically correct to use it where you'd use the other -u correlatives to refer to people.
-- Mi demandis tion al la politikisto. Tiu [politikisto] donis nur duonrespondon. - I asked the politician that. He ("thatbody") only gave a half-answer.