The Pronoun Ci
Is it just a way of specifying second person singular? Is it informal, like the German du or the French tu, or could it be used to address someone with authority?
Or, should one just keep clear of it?
Ci was not part of Esperanto as it was published in 1887. It's not part of Esperanto now. It's best ignored.
You should avoid it, but you may need to know what it means if you read early Esperanto texts. It's the familiar "you".
Is it specifically singular, as opposed to vi, which can be either singular or plural?
At Bertilo's grammar class at NASK this year, we briefly talked about why ci is worth avoiding. The big problem is that it means very different things to different people depending on their native language:
Is it supposed to be intimate?
Is it supposed to be friendly?
Is it supposed to be insulting?
Is it supposed to be the universal non-plural form of vi to be used in any second person singular situation?
Is it supposed to be an old-sounding pronoun, like how many English speakers consider "thou?"
This kind of confusion breeds misunderstanding and argument.
Estus pli bele, taŭge k logike diri k skribi "tvi"-n anstataŭ "ci" en situaĵoj kie oni alparolas personojn intimecajn. Unue, "tvi" estas kiel "vi", nur aldonante "t"-n enplie por la intimeco. Due en multaj lingvoj indoeŭropaj, inkluzive de la sanskr-ta, tvi aŭ tva aŭ tvam ekzistas aŭ estas rekonebla. Ci, male, ne similas nenion.
Estus pli bele, taŭge k logike diri k skribi "tvi"-n anstataŭ "ci"
For those reading along, please be aware that the above line is pure fiction - or at best, a matter of personal opinion with no connection whatsoever with how Esperanto is actually spoken. In 22 years, I don't think I have ever heard the word tvi. I suspect that Mirville2 simply made it up. For sure one should not use it with any expectation that the intended meaning will be understood.