I'm a bit confused here. I'm doing the "Kurso de Esperanto" course on Leciono 5. One question that I have to translate is: "What is this?". I've read different translations so far, from "Kiu estas tio?" to "Kio estas tio?" to "Kio estas tiu" and a couple others. What is the correct one and why do I see these other translations? What are the above phrases in English?

Hope that makes sense!
Dankon!

Kiu and tiu refer to things that are assumed to be known by both speakers - for example:

"Kiu estas tiu?" "Which of the things we are talking about is this that we are talking about?" (not a perfect translation, but this is mostly to show the nuances of -iu words)

However, kio and tio don't really have to be known or describing an assumed noun etc, so you could translate "Kio estas tio" as "What is that? (Hey, look at that unidentified object! I wonder what that could be!)".

So, "Kio estas tiu" could be "What is that (I have no clue what that thing we are talking about is)", and "Kiu estas tio" could be "Which of the things we are talking about is this unidentified object?"

That's not a perfect explanation, but hopefully you can see the slight differences in use.

I tend to translate "kio" as "what", and "kiu" as "which" or "who".

"What is that?" is different from "Which is that?". With "what", you don't even know what it is. With "which", you (probably) know what it is, you just want to know the specific one out of the set you see before you (which is why "kiu" can also mean "who", in the sense of "which person out of all of humanity?")

"Tio" and "tiu" follow the same exact pattern, except that they identify instead of ask about. Unfortunately, in English they are both covered by the word "that". In this case, English is less precise than Esperanto, which is probably why you are experiencing a difficulty with them. You need to learn the difference between "that thing over there" (tio) and "that one out of the group" (tiu).

Because "kiu" can also mean "who", "tiu" can also mean "that person", whereas you might otherwise use "tio" if you were talking about a dog or an airplane or some other nonhuman object. Slightly irregular, but, I suppose, a bit more respectful to people that we don't lump them in with other objects.

Also note that the word "ĉi" is used to turn "that" into "this".

What is that? = Kio estas tio?
What is this? = Kio estas ĉi tio?

there = tie
here = ĉi tie

that one = tiu
this one = ĉi tiu

etc