familinomo

A compound with one small mechanical lesson in it.Familio plus nomo gives familinomo, not familionomo: the -o of the first element drops, because in Esperanto compounds the leading root contributes its bare stem and the grammatical ending belongs to the last piece only. Where dropping it would leave an unsayable seam, an -o- goes back in, and here it would not, so it does not. Familia nomo, written as two words with the adjective, is equally correct and rather commoner in careful prose.

Now the fact that matters more than the build: surnames do not inflect. That is not a special rule for this word, it is the working habit of the language, and the personal names article sets it out with the evidence. Surnames are not Esperantised, in modern practice by anybody, and when only part of a name has been Esperantised only that part takes the accusative. Zamenhof wrote Poste la morto rabis al ni Jozefon Wasniewski, with the -n on the given name and nothing at all on the surname. PMEG records the same habit with Christenon Pedersen, and calls it a kutimo, a habit, rather than a rule.

So the practical instruction is short. Leave the surname exactly as its owner writes it, do not add -o to make it declinable, and do not add -n when it is the object. Mi konas Marion Smith, not Mi konas Marion Smithon.

The rest of the name vocabulary. Antaŭnomo or persona nomo is the given name, baptonomo is a Christian name specifically, kromnomo is a nickname, and plennomo is the whole thing. The all-capitals surname you see on congress badges is a typographic convenience for telling the family name apart when the reader does not know which order a culture uses. It is not a rule, because Esperanto has no compulsory capitalisation rules at all.