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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 Esperanto, all common nouns end in -o:

magiisto a wizard
provinco a shire
ringo a ring

The plural adds -j:

magiistoj wizards
provincoj shires
ringoj rings

Words that are not already nouns can be made into one simply by adding -o to the root. What sort of noun they will become depends on the sort of root they are; adjectival roots become abstract nouns, verbs usually (but not always) become single instances or results of their action, while adverbs, prepositions and numbers become more or less concrete manifestations of the root:

malico (from malica “malicious”) evil, malice
sorĉo (from sorĉi “to bewitch”) a spell
hodiaŭo (from hodiaŭ “today”) today, the present day
dekduo (from dek du “twelve”) a dozen
ekstero (from ekster “outside of”) an exterior

The type of root a word has is not always obvious — from brosi, for example, one might expect broso to mean “a brush stroke”, but it actually means “a brush”, because bros- is a noun root denoting the instrument, not the action. To make brushstroke from bros-, one must add a word like tir- (“pull”) and make brostiro. On the other hand, kombo does not mean “a comb”, because komb- is a verb root denoting the action; the instrument is a kombilo.

The -o of Esperanto nouns should not be confused with the masculine -o of Spanish and Italian; every noun in Esperanto, whether it’s male, female, neuter, or epicene, ends in -o. On the other hand, words that indicate kin relationships (and traditionally all living creatures) are male by default unless suffixed with -in- or prefixed with ge-:

frato brother
fratino sister
gefratoj siblings
koko* rooster/chicken
kokino hen
*Some words are ambiguously either male or epicene by default, depending on the speech habits of the speaker. One sometimes finds vir- (“man”) prefixed to such words to separate the roosters from the hens, for example, leaving koko to mean simply “chicken”.

It should be noted that ge- means “both sexes together”, and traditionally (and logically) could refer only to groups of mixed gender; one had to combine it with -an- “member” to refer to an individual (ex., gefratoj “siblings”, gefratano “sibling”). Only sometime after 1980 did the Plena Ilustrita Vortaro acknowledge the use of ge- with singular nouns to convey “one of either sex”, though it otherwise still means “both sexes together”, and is not used as a general epicene prefix for anything else. (One uses koko for “chicken”, for example, not gekoko.)*

*One can find -iĉ- proposed throughout the internet as a male suffix, which would render words that are ambiguously male or epicene unequivocally epicene (ex., amiko would only mean “friend”; “male friend” would be amikiĉo). Words like patro and edzo, on the other hand, which indicate a male by definition, would, in less radical versions of iĉismo, be unaffected, but would require the intro­duction of epicene counterparts to which one could affix gender suffixes if one wanted.

For all its popularity among neologists, however -iĉ- has yet to gain official status — and was discouraged by the Plena Manlibro de Esperanta Gramatiko until 2016, when it acknowledged -iĉ- as a popular though little used proposal.

Proper Nouns

Proper nouns name a particular person, place, or thing, and in Esperanto have posed a problem since the beginning.

At the heart of the trouble is Esperanto’s accusative ending -n, which requires that names, like any other noun, end in a vowel. The free word order possible in Esperanto makes the accusative case difficult to avoid — without it, one could not tell who is doing what to whom — which means that either all names must be forced to end in a vowel, even if only when used as the direct object, or Esperanto’s more conventional subject-verb-object default word order must be observed while using “indeclinable” names.*

*Theoretically, there’s a proposed preposition na to mark the accusative case for indeclinable nouns, as in na Pussy Riot malamas Putin “Putin hates Pussy Riot”, but I’ve never personally seen na used anywhere.

For many names, this is not an issue — most biblical and classical names, along with many familiar modern European ones, have more or less offical Esperanto equivalents; Gaius Julius Caesar, for example, is Gajo Julio Cezaro, where each name has been Esperanto-ized.

As for names without an Esperanto equivalent, those originally written in the Roman alphabet (including Latin renderings of Greek and Biblical names) are often transcribed as-is (ex. Barack Obama); those written in other alphabets are transcribed phonetically (ex., Nikita Ĥruŝĉov).

The names of countries, oceans, and international rivers and mountain ranges more or less preserve their Latin (or Latinized) form, but conform to Esperanto’s orthography and have, where necessary, been altered for the sake of regularity:

Afriko Africa
Ameriko America
Azio Asia
Eŭropo Europe
Ĉinio China
Kostariko Costa Rica
Peruo Peru
Rusio Russia
Usono* The United States
Mediteraneo The Mediterranean Sea
Pacifiko The Pacific Ocean
Balkano The Balkan Mountains
*From a 19th-century acronym for “United States of North America”

In English, the name of a country’s inhabitants is sometimes the basis for the name of the country and language (e.g., “England” and “English” from the ancient Ængle), sometimes the other way around (e.g. “Congolese” from “Congo”). In the latter case, the language might instead be derived from the ethnic group whose language it is (“Spanish” in the case of Mexico), or have its own name (“Swahili” in the case of Kenya).

Esperanto follows a similar model: anglo (“an Englishman”) forms the country name by adding -io* to the root (Anglio) and the language name with la -a (e.g., la angla “the English language”), but kongano (“a Congolese”) and la kongana (“Congolese language”) are derived from Kongo by adding -an- to the root. The language of Mexico is la hispana (“Spanish”, from hispano “a Spaniard”); the language of Kenya is la svahila.

*or -ujo and sometimes -lando for orthodox Esperantists before 1975, when popular -io was officially no longer discouraged
japano a Japanese
la japana Japanese (language)
Japanio Japan
Brazilo Brazil
Brazilano a Brazilian
la brazilportugala Brazilian Portuguese (language)

The names of states, provinces, and cities, most of which don’t have common Latinized or Latinizable names like many countries do, are more or less treated like personal names; some larger cities and well-known states have Esperanto-ized names (ex., Nov-Jorko, Kalifornio), but most are either re-written with Esperanto’s orthography or left as-is.