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A noun is any sort of person, place, or thing, and comes in two varieties: common and proper.

Common Nouns

Common nouns are generic words that identify members of a class of people, places, or things. In Ido, all common nouns end in -o:

sorcisto wizard
provinco shire
ringo ring

The plural changes the final -o to -i:

sorcisti wizards
provinci shires
ringi rings

Words that are not already nouns can be made into one simply by adding -o to the root:

maligno (from maligna “malignant”) evil person or thing
sorco (from sorcar “bewitch”) incantation
extero (from exter “outside of”) outsider

The -o of Ido nouns should not be confused with the masculine -o of Spanish and Italian; every noun in Ido, whether it’s male, female, neuter, or epicene, ends in -o. So to specify that a noun is male or female, one can add -ul- or -in- to the root:

frato sibling
fratulo brother
fratino sister
hano chicken
hanulo rooster
hanino hen

Note that -ul- and -in- should only be used to avoid potential confusion. When speaking about gesiori Smith (“Mr. and Mrs. Smith”, the prefix ge- specifying both Mr. and Mrs. together), for example, one might need to distinguish between siorulo Smith (“Mr. Smith”) and siorino Smith (“Mrs. Smith”), but not when addressing either one of them directly (both are Sioro Smith).

Proper Nouns

Proper nouns name a particular person, place, or thing, and as such have no generic ending; they are treated as immutable “foreign” loanwords, pronounced as closely as one can get to the original within the limits of the Ido phonetic system. Those originally written in the Roman alphabet are transcribed as-is; those written in other alphabets are transcribed phonetically. Such words include names of individual people as well as words that are exclusively national or local:

Gaius Iulius Caesar GÁIVS·IV́LIVS·CAESAR
Sokrates SÓCRATÉS / Σωκράτης
Shakespeare Shakespeare
Khrushchov Хрущёв
Wałęsa (o Walesa) Wałęsa
dollar, dollars dollar, dollars
pound, pounds pound, pounds
samurai samurai / 侍
Beijing Běijīng / 北京
Lisboa Lisboa
New-York New York
München (o Munchen) München
Moskva Москва

Note that if the plural of a foreign word is known, it should be used as the plural in Ido, too (eg., dollars). If it is not, one should simply add -i (including the hyphen) where necessary.

Unlike other “foreign” words, biblical and Christian given names do undergo some Ido-izing in that hard c is transcribed k and ph is transcribed f — but otherwise adhere to their Latin or Latinized forms of the nominative case:

Iakobus Jacob, Giacomo, Jaques, etc.
Adolfus Adolf, Adolph, Adolfo, etc.
Rafael Raphael, Rafaelo, Rafaele, etc.
Ioannes John, Johann, Jean, Giovanni, Ivan, etc.

The rules of Ido-izing such names are not consistent, however — sometimes something other than the Latin nominative case is used, sometimes the Latin termination is left off, sometimes intervocalic s is transcribed s, sometimes z:

Iesu IÉSV́S / ישוע
Isaias ÍSÁIÁS / יְשַׁעְיָהוּ
Iozef IÓSÉPHVS / יוֹסֵף

The names of countries, oceans, and international rivers and mountain ranges preserve their Latin (or Latinized) form, but are more aggressively Ido-ized in their orthography and regularization:

Afrika Africa
Amerika America
Kanada Canada
Azia Asia
Europa Europe
Chinia China
Kostarika Costa Rica
Peru Peru
Rusia Russia
Usa* The United States
Mediteraneo The Mediterranean Sea
Pacifiko The Pacific Ocean
Balkani The Balkan Mountains
*From “United States of America”.

Most “Old World” country names are derived from the name of their dominant ethnic group, as England is from the English and Russia from the ancient Rus’; most “New World” peoples derive their name from the name of their country, as Canadians do from Canada. The names of the inhabitants of the first group are generally formed in Ido by replacing the final -ia of the country name with -o; the names of the inhabitants of the second group are formed by adding -ano to the country name’s root:

Anglo Englishman
Anglia England
Mexikia Mexico
Mexikiano Mexican