When should I use "tio" and "tiu"?

Question

I have two examples from Duolingo that puzzle me:

Whose is that? Kies estas tio? Whose car is that? Kies aŭto estas tiu?

Why is tiu used in the second example? Why does the introduction of the car make a difference? Is there something else implied in the tiu question, such as Whose car is that [one of multiple cars]?

Is there a case where you would ask Kies aŭto estas tio?

Answer 1

Use tio when you don't know the nature of what you are pointing or if you don't want to mention its nature. (By "nature", I mean every kind of concept, such as cars, dogs, ideas ...)

You can read "tio" as "that thing/situation".

Kio estas tio? = What is that? / What thing is that thing?

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Use tiu when you know the nature of what you are pointing.

Kio estas tiu aŭto? = What is that car? / What thing is that car?

Here, the nature is a car. It is often present in the same sentence, often just after tiu, or any correlative that ends with -u.

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Note 1: Be careful, because some Esperantists tends to use "tiu" without mentionning the nature in the same sentence. They do this because they think the nature is obvious because of the context or if they already mentionned it in past sentences. So, you might see something like that:

Estas multaj aŭtoj. Kio estas tiu? = There are a lot of cars. What is that (car)?

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Note 2: When a correlative with -u (such as tiu) is used without mention of a nature of any sort, the nature is likely to be homo(j) = person/people.

Ĉiu volas lerni Esperanton. = Every person wants to learn Esperanto.

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Note 3: If you are a beginner, avoid doing what is mentionned in Note 1 and Note 2.

Answer 2

Another difference is that tio can't be used before other nouns. Tio aŭto isn't right, but tiu aŭto is.

Answer 3

Kio and tio take the place of a noun in a sentence. Kiu and tiu point to a specific noun in the sentence. Any time you see kiu or tiu, there should also be a noun with it. Note: sometimes the nouns are implied. To put it simply, Tio and kio replace the noun, whereas tiu and kiu point to the noun.

Kio vi estas? = What thing are you? The answer would be that you are a human.

Kiu vi estas? = Which person are you? The answer would be your identification (example, your name). In this example, the noun persono is implied and omitted. The full sentence would be Kiu persono vi estas?

Kies estas tio? = Whose is that thing? - In this case tio is taking the place of a noun.

Kies aŭto estas tiu? = Whose car is that? - In this case tiu is pointing to the noun of the sentence, which happens to be aŭto.