Esperanto nouns use a visible sequence of endings. The ordinary dictionary form ends in -o: libro “book,” amiko “friend,” ideo “idea.” Add -j when the noun refers to more than one countable unit, and add -n when its sentence role requires the accusative. The order is fixed: root + o + j + n.
This is a productive system, not a collection of irregular plurals. The root remains stable while the endings state word class, number, and case. Once you know libro, you can predict libroj, libron, and librojn without memorizing four separate entries.
The four basic forms
| Form | Structure | Core use | Example |
|---|---|---|---|
| libro | libr-o | singular, non-accusative | one book as subject or after a preposition |
| libroj | libr-o-j | plural, non-accusative | multiple books as subject |
| libron | libr-o-n | singular accusative | one book as direct object |
| librojn | libr-o-j-n | plural accusative | multiple books as direct object |
Libro kuŝas sur la tablo.
A book is lying on the table.
Libroj kuŝas sur la tablo.
Books are lying on the table.
This minimal contrast changes only -j, so it changes one book into multiple books. Esperanto has no irregular equivalents of English children, mice, or feet: infano → infanoj, muso → musoj, piedo → piedoj.
The category role of -o is introduced fully under nouns and the -o ending. Here the focus is how number combines with that citation form.
When -j is required
Use -j when you mean multiple countable individuals, even if another word already gives the number. Esperanto does not let du carry plurality alone: say du domoj, not du domo. The same applies after multaj, kelkaj, and quantity phrases such as multe da.
Du gastoj jam atendas ĉe la enirejo.
Two guests are already waiting at the entrance.
Kelkaj infanoj ludas en la ĝardeno.
Several children are playing in the garden.
Ni havas multe da demandoj, sed ne multe da tempo.
We have many questions, but not much time.
The last example contrasts a count noun, demandoj, with a substance-like quantity noun, tempo. A plural meaning elsewhere in the phrase does not cancel the noun’s own -j when distinct countable units are intended.
“Kiom da biletoj vi bezonas?” — “Mi bezonas tri biletojn.”
“How many tickets do you need?” — “I need three tickets.”
The answer has both plural -j and accusative -n, producing bilet-o-j-n. The detailed object rule is covered in the accusative -n overview.
Accusative after number
Number and case are independent. -J says how many referents there are; -n says what role the noun phrase plays. A plural subject has -j without -n. A plural direct object normally has -jn.
La studentoj legas la artikolojn.
The students are reading the articles.
La artikoloj interesas la studentojn.
The articles interest the students.
The forms swap roles while remaining plural. In the first sentence, studentoj is the subject and artikolojn the object. In the second, artikoloj is the subject and studentojn the object. This explicit morphology helps Esperanto permit more word-order variation than English.
Adjectives inside the same noun phrase normally agree in both number and case:
Mi aĉetis du freŝajn panojn por la pikniko.
I bought two fresh loaves for the picnic.
Both freŝajn and panojn carry -j-n. The numeral du does not take those endings. Agreement receives its own treatment under adjective agreement in number.
Singular form does not always mean one object
Many -o nouns denote substances or non-counted continua: akvo, lakto, sablo, oro, muziko. Their ordinary unmarked form does not assert “exactly one water” or “one music.” It presents the referent without dividing it into countable units.
Bonvolu alporti akvon kaj panon.
Please bring water and bread.
Ne restas lakto en la fridujo.
There is no milk left in the refrigerator.
Use measurement nouns when you count portions: du litroj da akvo, tri tasoj da kafo. The units take -j; the measured substance normally does not.
Ni mendis tri tasojn da kafo.
We ordered three cups of coffee.
Substance nouns can take -j when the intended units are different kinds or recognized varieties: italaj vinoj means several wines or kinds of wine. That meaning shift is regular, but it is not the ordinary way to express a larger quantity of one substance.
La vendejo ofertas lokajn fromaĝojn kaj vinojn.
The shop offers local cheeses and wines—several varieties or products.
English countability is only a clue, not a law. Esperanto meblo is one item of furniture and mebloj multiple items; konsilo/konsiloj counts pieces of advice; mono normally remains non-counted even though it consists physically of coins and notes.
The full set of shifts belongs under count and mass meanings.
Groups and collective singulars
A singular noun can name one group containing many members: familio, teamo, klaso, arbaro. Its grammatical number counts groups, not internal members. Unu familio may contain five people; tri familioj means three separate family groups.
La tuta familio venos morgaŭ.
The whole family will come tomorrow.
Tri familioj loĝas en tiu granda domo.
Three families live in that large house.
English sometimes allows plural verb agreement with a collective noun, especially in British usage. Esperanto verbs never agree for number at all, so la familio venos and la familioj venos use the same verb form. The noun ending alone shows whether one or several groups are meant.
Derivation from a stable root
The word family makes the system visible. From vojaĝ- come vojaĝi “to travel,” vojaĝa “related to travel,” and vojaĝo “a journey.” Then vojaĝoj refers regularly to multiple journeys. The -o is not fused permanently into an unanalyzable English-style stem.
La vojaĝoj de ŝia infanaĝo inspiris ŝian unuan romanon.
The journeys of her childhood inspired her first novel.
Not every mechanically imaginable derivative is equally common. Morphology makes a form interpretable; established usage decides whether speakers actually prefer it. Consult a dictionary for lexical convention while trusting -o/-j/-n once a noun is established.
Common Mistakes
1. Leaving the noun singular after a number greater than one. The numeral does not replace -j.
❌ Mi rezervis du ĉambro.
Incorrect — two countable rooms require plural -j.
✅ Mi rezervis du ĉambrojn.
I reserved two rooms.
2. Putting -n before -j. Number always precedes case.
❌ Ŝi legis la libronj.
Incorrect — the ending order cannot be -n-j.
✅ Ŝi legis la librojn.
She read the books.
3. Marking only the adjective or only the noun for plural. Ordinary attributive adjectives agree.
❌ La novaj studento alvenis.
Incorrect — the plural adjective and singular count noun disagree.
✅ La novaj studentoj alvenis.
The new students arrived.
4. Copying English mass behavior mechanically. Esperanto can count concepts that English packages as mass nouns.
❌ Ŝi donis al mi du bona konsilo.
Incorrect — two pieces of advice are two countable konsiloj, with adjective agreement.
✅ Ŝi donis al mi du bonajn konsilojn.
She gave me two good pieces of advice.
5. Using a plural substance noun merely to mean “a lot.” Use a quantity expression unless distinct types are intended.
❌ Mi trinkis multajn akvojn post la kuro.
Wrong for the intended meaning ‘a lot of water’—akvoj suggests different waters or varieties.
✅ Mi trinkis multe da akvo post la kuro.
I drank a lot of water after the run.
Key Takeaways
- The predictable noun forms are -o, -oj, -on, -ojn, always in that order.
- Countable nouns take -j for multiple units even after numerals and plural quantifiers.
- Number and accusative case are independent, so plural objects end in -jn.
- Substance and abstract nouns normally lack -j unless multiple types or bounded instances are intended.
- The stable root supports derivation; -o is a productive noun ending, not an inseparable part of every stem.
Related Topics
- Nouns and the -O EndingA1 — Build Esperanto nouns with -o, then add -j for plural and -n for accusative while respecting meaning and countability.
- Count and Mass MeaningsA1 — How Esperanto distinguishes countable individuals from unbounded substances and shifts nouns between those meanings.
- Plural Meaning without -JA2 — When a singular Esperanto noun can denote a group, kind, or undivided quantity containing more than one element.
- Adjective Agreement in NumberA1 — Learn why Esperanto adjectives take -j when they describe plural people or things, in both noun phrases and predicates.
- The Accusative -N: OverviewA1 — Understand how Esperanto uses final -n to mark direct objects, direction, time, and measurement without relying on word order.