If you read the PIV definitions, you will see that voli is (or was) primarily about volition: i.e. will/intention. Deziri is about desire, wishes. I think, frankly, that many of the sentences here at duolingo (though not necessarily this one) are currently much over-using voli for both cases. This probably reflects common usage, but I personally think it is a little sloppy and I am submitting alternative answers for all these cases.
Apparently, "He wants, that I receive his letter" isn't accepted, likely because it's unnatural English, even though it's a literal translation.
I was just thinking, while I wrote it, how very much it sounded like something a native Yiddish speaker (like Zamenhof) would say. More common would actually be "He wants, that I should receive his letter" (calqued from "Er vil, az ikh zol bekumen zayn brif").
Even though I speak this way myself, I'm not really sure if it should be added to the acceptable answers, since it's not standard English.
You are correct that this is unnatural English.
To start with, the comma is incorrect in English. In Esperanto this sentence is two clauses, but in English it is only one clause.
You could say "He wants," "He wishes," or "He desires," but no matter which way you say it, "that" will not sound natural as the next word in this sentence. Instead of "that I," you could say "for me to," or just cut out that word entirely and say "He wants me to receive his letter."
I won't mention what you said about how Zamenhof would speak, because I don't know much about Yiddish, but I do feel the need to say that in many cases you can't translate literally. This is what Google Translate does for most languages, and is why it has such a reputation for being untrustworthy. This is what makes learning another language so hard; instead of learning the translation for each individual word, you have to learn ideas, how to put them together, and eventually how to create your own. It's tough, but it's worth it
I don't think the difference is the number of clauses. That should be the same. I think it's just about comma use. English doesn't use commas to separate dependent clauses; Esperanto does.
Also, "he desires that" isn't unnatural English, though it is more formal than typical spoken English. https://ludwig.guru/s/he+desires+that
That's an easy mistake to make but no. If he wanted his own letter then you would, indeed, say "Li deziras sian leteron". But in the sentence above, he's wishing that you receive his letter. What follows the "ke" is, I believe, a sub-clause, which must be grammatically correct by itself, namely "mi ricevu lian leteron" (let me get his letter). All together, it must be "Li deziras ke mi ricevu lian leteron".
No. "Li deziras" is a separate clause from "ke me ricevu lian leteron" and "sian" only works within the clause. If you were to use "sian," it would mean "He wishes for me to receive my letter." "Li" in this case is referring to the same person as "li." The lack of context is one thing I dislike about Duo exercises, but I don't know of an easy fix for that
Oh. Yeah, you could say it with either of those. I think there are some subtle differences, tho, as there are in English.
You can use the conditional tense to say "La vetero estus bona se ne pluvus." (The weather would be good if it weren't raining.) You can use it without a second clause, but one can infer that there is still an unspoken one there. For example, "Mi esperas ke la vetero estus bona" (I hope that the weather would be good) can imply "sed mi dubas tion" (but I doubt that it will be).
The future tense can also be used. In a statement, "Morgaŭ, la vetero estos bona" (Tomorrow, the weather will be good) it's definitive. It also works when expressing a hope: "Mi esperas ke morgaŭ la vetero estos bona." (I hope that tomorrow the weather will be good.) This is perfectly acceptable.
But the imperative form, in addition to commands and requests, can also be used to show a want/desire/wish/hope. It gives it a little more oomph. "Mi esperas ke la vetero estu bona" (I hope that the weather be good) sounds like you're willing the weather to be good.
Maybe someone was on holiday and wrote a really lovely letter while he was away but the snail mail was so slow that the letter still hadn't arrived when he got back home. It has been so long that he is afraid the letter may have gotten lost in the mail but he still wants me to receive his letter.
Ok, now I'm completely confused about who "lian" or "sian" refer to when used in a ke-clause. 1) Adamo deziras, ke Pedro recivu lian leteron. 2) Adamo deziras, ke Pedro recivu sian leteron. Which one means Adam wants Peter to receive Adam's letter? Pedro's letter? A third fellow's letter?
Mail is tricky because there are two people involved in every letter, the sender and the receiver. And we can say the letter belongs to either party depending on who we want to emphasis.
Let's simplify a little:
"Pedro recivis sian leteron."
Here, the pronoun refers to Pedro. (Sian always refers to the subject of the clause.) We are talking about a letter received by Pedro with no reference to who is sending it. There is sender, but we don't really care.
"Pedro recivis ĝian leteron."
In this one, the pronoun refers to the sender. (Ĝian, lian, ŝian, and ilian never refer to the subject of the clause.) We are still talking about a letter received by Pedro, but this time we care (a little) about the sender.
"Pedro recivis lian leteron."
Now, we are explicitly picking the male half of the world. It could be Adam, it could be Dave, it could be Joe; but it's not Sophia or Lidia.
"Pedro recivis leteron de Adamo."
Finally, we are explicitly naming Adam as the sender.
In contrast, here are some examples where there is no ambiguity about the pronouns:
"Pedro kisis sian koramikino."
Pedro kissed his own girlfriend.
"Pedro kisis lian koramikino."
Pedro kissed somebody else's girlfriend.
Whew. Now, on to the ke-clause!
"Adamo deziras, ke Pedro recivu sian leteron."
Since the sian/lian distinction occurs within a clause, the sian is still saying "letter received by Pedro" and has nothing to do with Adam--it neither confirms nor eliminates him as the sender. (We'll have to depend on context for that.)
"Adamo deziras, ke Pedro recivu lian leteron." Now, the lian does not refer to Pedro; instead, it is referring to the sender. It could refer to Adam, since Adam is not the subject of the clause where the lian occurred; but you can't be positive. It could be that Adam wants Pedro to receive a letter from Pedro's father. (And that the letter contains enough cash for Adam and Pedro to go bar-hopping on Friday night!)
"Sofia deziras, ke Pedro recivu lian leteron." In this case, because of the mismatch between Sofia and lian (unless Sofia isn't telling us something...), we can say that we are talking about a third-party sending the letter.
And for the contrast:
"Adamo vidis ke Pedro kisis sian koramikino."
Adam saw that Pedro kissed his own girlfriend.
Again, the sian refers to the subject of the clause, Pedro, and has nothing to do with Adam even though Adam is the subject of the entire sentence, since Adam is outside of the ke-clause.
"Adamo vidis ke Pedro kisis lian koramikino."
Adam saw that Pedro kissed somebody else's girlfriend.
The lian does not refer to Pedro, because it never refers to the subject of the clause. It could refer to Adam, even though Adam is the subject of the entire sentence, because Adam is outside of the ke-clause. However, it could also refer to some third person.
Part of your problem might be a lack of understanding of the volitive case which Zamenhof rolled into the Imperative for simplicity's sake. The volitive essentially says "I (s/he, you, we) want this to happen" but isn't intended to strictly command that it Shall be done! In this instance it is obvious that the intention is that something should happen, but nobody is demanding that it shall.
If it might help you Lernu.com has a pretty good course, though it may take a while to get there, or find a copy of "Being Colloquial in Esperanto," or "Plena Manlibro de Esperanto Gramatiko," either in hardcopy or on-line, and look in them for the relevant information.
Dankon por cxi tio. Another source of confusion is having seen "voli" used both with and without the volitive case, and seeing "deziri" translated as both "want" and "desire". I suppose that's just a matter of reading a lot of Esperanto and getting accustomed to the usages . . .
I think that Duo is a bit sloppy about those two words. Voli comes from the Latin volere and can be traced back to the Hindo-European root wel, "To will, wish for, want." Zamenhof chose to give it the meaning of "Wish."
Deziri comes by way of the French and English from the Latin Desiderare which broke into de (from) and sidis/sideris (star, astrological item), which Zamenhof decided meant "desire."
Bezoni was given the meaning of "to want." From the French besoin = "Need, want, necessity." But, as I'm certain that you can see, this was the wrong "want" for this need.
When I was first learning the language I learned voli as "to want, will (something)" and deziri for "To wish, desire (for something)" Fairly different meanings that seem to have become mushed together, probably for very sound etiological reasons, but leading to unneeded confusion.
I'm occasionally asking the Owl, politely, to go back to those initial meanings and cease the confusion.