Common nouns are generic words that identify members of a class of people, places, or things. In Esperanto, all common nouns end in -o:
The plural adds -j:
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-:
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.)*
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 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.
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:
|Usono*||The United States|
|Mediteraneo||The Mediterranean Sea|
|Pacifiko||The Pacific Ocean|
|Balkano||The Balkan Mountains|
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 and languages of the first group are generally formed in Esperanto by replacing the final -io* of the country name with -o and -a, respectively; the names of the inhabitants and languages of the second group are formed by adding -ano to the country’s name and changing the last vowel of the name to -a:
|la angla||English (language)|
|Usono||The United States|
|la usona||American English (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.