You might never encounter the pronoun “ci” in your adventures as an Esperantist, unless you happen to be reading some old-fashioned literature. “Ci” is much like the antequated English word “thou” (which would make “cia” the equivalent of “thine,” and the accusative “cin” something like “thee”). I have no idea how you could work that into your everyday parlance. Perhaps if you were among a bunch of medieval era enthusiasts?

Anyhow, it’s worth knowing that “ci” exists. If you ever translate pieces of older literature, you may find yourself in need of it. Also, if you happen to be translating from a language that differentiates between formal and informal address in its pronouns (like Spanish, with its varied “tu” and “usted” forms), you might want to consider using “ci” to preserve the authenticity of the original address. Of course, “ci” follows all the rules of Esperanto grammar, so you would apply all the same rules to it that you do your other pronouns.