Put it like this:
The stress is always on the second-to-last vowel.
The vowels in Esperanto are: a, e, i, o, u.
(The consonants are: b, c, ĉ, d, f, g, ĝ, h, ĥ, j, ĵ, k, l, m, n, p, r, s, ŝ, t, ŭ, v, z.
Note that both j and ŭ are consonants! Example: ankaŭ has the stress on the first a - Ankaŭ - not "ankAŭ".)
To answer your question:
Skip "syllables". Count the vowels (a, e, i, o, u) instead.
tiu has the stress on i
tiuj also has the stress on i
I believe that when the -j plural marker is added on, the syllable to which it is attached becomes a diphthong, meaning it is still one syllable with a sort of 'blended together' sound. So if "tiu" is pronounced like "TEE-oo", the word "tiuj" would then be pronounced "TEE-ooee".
A similar effect occurs with the 'a' and 'ŭ' in "aŭto", or just "aŭ". Alone, the 'a' is just an "ah" sound, but when 'ŭ' is added to it, there's still only one syllable, but pronounced together like "ow" (as in "out").
The primary difference between -iu words and -io words is that an -iu word can come before a noun while an -io word must be by itself. So ‘tio’ means ‘that’ while ‘tiu aŭto’ means ‘that car’.
It gets a little confusing because an -iu words can stand by itself, if the noun is implied. So is someone asks ‘Kiu aŭto?’ (which car?), you can point and reply just ‘Tiu’ (that one). But unless there's something that I haven't learnt yet, you can never say *‘Tio aŭto’.
Then it gets even more confusing, because often ‘homo’ is the implied noun even when it never appeared. So ‘Kiu prenis mia aŭto?’ means ‘Who [which person] took my car?’, because presumably it has to be a person. And while theoretically ‘Kio prenis mian aŭton?’ would make sense, it seems dehumanizing, just like ‘What took my car?’ would be in English, so you use ‘kiu’ instead. (But if you suspect that it was taken by a machine or an animal, then you could say ‘kio’).
So this is how we get the idea that -iu words are for people while -io words are for things. But really, that is only derived from the main difference that I mentioned at the beginning of this comment; and if there's a noun following it, then you always use an -iu word, whether it's a person or not.
(Although some people do use the plural forms, nonstandardly.)
Something must be in the air. In a different thread someone else claimed that there are situations where the -io correlatives can be plural. My sense is that it's not that people pluralize it nonstandardly, but rather that they are still learning -- or have decided to make up their own language. I've seen a lot more in the former category... if any in the latter.
In fact, you can -- you just can't do it in the Duolingo course. The course makes a very strict difference between "this" and "that" which is not always observed in reality.
Occasionally when I say this, my comments will get voted down (presumably by people who have learned in the course that "Ĉi tie" means "here" not "there"), but nobody has yet to give an example of a single appearance of a demonstrative word like this, appearing by itself with no other demonstratives as contrasts, where this distinction matters.
I can point to a spot right in front of me on a table and say "look there, there's an ant on the table!" and nobody will question it.
So, back to the original sentence, if I were to come up to a bunch of cars for sale, I could very easily drop the ĉi. But, if the conversation was more like this:
- Ĉu tiuj aŭtoj estas bonaj?
- Ne, ili estas malbonaj, sed ĉi tiuj aŭtoj estas bonaj.
... in this case, you can't drop the ĉi.
You've probably already figured this out, but for anyone else first seeing this question, there are indeed more specific terms: veturilo for a vehicle (from veturi, to ride), kamiono for a truck, kamioneto for a van, taksio for a taxi, and aŭtobuso for a bus, to name a few.
See here for more info on correlatives: https://en.m.wikipedia.org/wiki/Esperanto_vocabulary#Correlatives
I don't understand your reference to "people and things". Is that in the Tips and Notes? It sounds like you're quoting something, but I can't tell what. Here's what you need to know to see how this works:
Point 1: Tio (and related words like io, kio, ĉio, and nenio) will take an -n if they are a direct object, but they never take a -j ending.
Point 2: Tio answers "what" and tiu answers "which" or "who".
Keep in mind also that we sometimes say "what car" when we really mean "which car."
So - which cars are good - those cars are good. It needs to be a form of tiu.
Another way to put all this is that if "this" comes before a noun (expressed or implied) it needs to be a form of tiu.
In the course, yes.
In real life, probably not.
It's not unlike in English. When there's only one thing, there's no real difference between "what is this" and "what is that" because by definition, one thing can't be "closer" to the speaker - since "closer" implies the comparison of two things.