Esperanto can present the same numerical idea in several grammatical forms. Bare du counts two things; noun duo names the number, digit, pair, or group of two; adjective dua identifies second position; adverb due places something second in an ordered account. The ending is therefore not decorative: it changes both the word class and the relation expressed.
This productive system is more regular than English two, pair, second, secondly, but it is not mechanical in every detail. A derived form must have a meaning that makes sense in context, and established suffixes such as -on-, -obl-, and -op- add meanings beyond a simple change of ending.
Bare cardinals: quantity without agreement
The basic cardinals unu, du, tri, kvar, and so on are special invariant particles. Before a noun they answer kiom? “how many?” They never take plural -j or accusative -n in this use; the noun and any ordinary adjective carry those endings.
Ni mendis tri grandajn picojn por la tuta grupo.
We ordered three large pizzas for the whole group.
Ĉu vi vere legis ĉiujn kvarcent paĝojn dum unu nokto?
Did you really read all four hundred pages in one night?
Compare the unchanged tri with plural accusative grandajn picojn. English speakers sometimes try to make the numeral agree, but Esperanto keeps the quantity word stable.
A cardinal can also stand alone when the missing noun is obvious. In that position it behaves semantically rather like a noun, but it still does not acquire -o, -j, or -n:
— Kiom da seĝoj ni alportu? — Ses sufiĉos.
— How many chairs should we bring? — Six will be enough.
Kvin kaj sep faras dek du.
Five plus seven makes twelve.
Noun forms: duo, deko, cento
Adding -o creates a normal noun. A numerical noun can name a number or digit, a score or grade, or a group characterized by that number. Since it is a noun, it may take -j and -n.
| Form | Typical meaning | Example phrase |
|---|---|---|
| unuo | the number or digit one; one unit | mezurunuo |
| duo | the number two; a pair or duo | muzika duo |
| deko | the number ten; a group of ten | deko da ovoj |
| cento | a hundred as a grouped quantity | centoj da mesaĝoj |
La ĵurio donis al la filmo du naŭojn kaj unu dekon.
The jury gave the film two nines and one ten.
Ni aĉetis du dekduojn da glasoj por la kafejo.
We bought two dozen glasses for the café.
Here naŭojn, dekon, and dekduojn are ordinary accusative nouns. A number noun used as a quantity head normally connects the counted material with da: deko da ovoj, cento da paĝoj. This differs from bare dek ovoj and cent paĝoj, where the cardinal directly counts the noun.
For a multiword number, the parts become one derived noun. Hyphens can preserve the internal boundaries: dudek-trio “the number twenty-three” and du-mil-kvincento “the number 2,500.” These forms are possible but less frequent than simply using the bare number or digits. Do not invent a heavy noun form when no discourse need justifies it.
Miliono and miliardo are already nouns, not bare cardinal particles. That is why one says du milionoj da homoj and mi investis du milionojn da eŭroj. By contrast, invariant mil directly modifies the counted noun: du mil homoj.
Adjective forms: order and other numerical properties
Adding -a directly to a bare cardinal normally creates an ordinal: unua, dua, tria, centa. An ordinal answers kioma?—which position?—not kiom?—how many? Because it is an adjective, it agrees in number and case.
Mi legis la unuajn du ĉapitrojn en la trajno.
I read the first two chapters on the train.
El la kvar proponoj la komitato elektis la trian.
Of the four proposals, the committee chose the third.
The minimal contrast is exact:
Ni rezervis du ĉambrojn, sed ni dormos en la dua.
We reserved two rooms, but we will sleep in the second one.
In du ĉambrojn, the cardinal gives quantity. In la dua, the adjective identifies one position and stands for la dua ĉambro.
Other numerical adjective families contain an additional suffix. Duona means “half,” duobla means “double/twofold,” and duopa describes grouping by twos. Their meanings come from -on-, -obl-, and -op-, not from -a alone:
Ni faru duonhoran paŭzon post la prezento.
Let's take a half-hour break after the presentation.
La nova fenestro havas duoblan vitron.
The new window has double glazing.
These adjective forms follow normal agreement. Compare duonhora paŭzo, duonhoraj paŭzoj, and ni faris duonhoran paŭzon.
Adverb forms: sequence, fraction, and multiplier
The -e form built directly from a cardinal is normally an ordinal adverb: unue “first/in the first place,” due “second/in the second place,” trie “thirdly.” It orders actions, arguments, or stages rather than counting them.
Unue konservu la dosieron; due fermu la programon.
First save the file; second, close the program.
Ŝi alvenis due, nur tri sekundojn post la venkinto.
She arrived second, only three seconds after the winner.
Adverbs with added suffixes express different relations. Duone means “by half/partly,” duoble means “two times as much,” and duope means “in pairs/as a group of two.”
Mi nur duone aŭdis la anoncon pro la bruo.
I heard only half of the announcement because of the noise.
La nova kuirilo varmigas la akvon duoble pli rapide.
The new cooker heats the water twice as fast.
La infanoj eniris duope, por ke neniu perdiĝu.
The children entered in pairs so that nobody would get lost.
Dates, hours, years, and units
Several practical conventions follow from the same form–meaning distinction:
| Context | Form | Reason |
|---|---|---|
| quantity | tri minutoj | cardinal count |
| measured duration | tri minutojn | the noun carries extent -n |
| day of a month | la tria de Majo | ordinal position among days |
| clock hour | la tria | ordinal position in the cycle |
| year label | la jaro 2026 | identifying number; cardinal or ordinal reading varies |
When an event occurs on a date, the understood tagon commonly leaves accusative on the ordinal: la 3-an de Majo. A clock time instead normally has a preposition: je la tria. This contrast keeps a date and an hour distinct.
La trejnado okazos la trian de Majo je la tria posttagmeze.
The training session will take place on May third at three in the afternoon.
Measured extent puts case on the unit noun, never on the bare numeral:
La tablo estas du metrojn longa kaj unu metron larĝa.
The table is two meters long and one meter wide.
With digits, show adjective and case endings when the syntax needs them: la 4-a ĉapitro, mi legis la 4-an ĉapitron, la 12-an de Junio. A plain label remains plain: ĉambro 204, la jaro 2026. See dates and years and telling clock time for the full conventions.
Foundational norm, variation, and practical choice
The distinction between invariant basic cardinals and regularly derived -o/-a/-e forms belongs to the foundational grammar. The productive suffix families are also established Esperanto word formation. Variation appears mainly in spelling long derived numbers and in reading year labels: a spaced or hyphenated ordinal can coexist with a cardinal-style reading of a year.
Corpus usage confirms that short, useful forms—duo, unue, duone, duoble—are common, while enormous derived number nouns are naturally rare. The practical recommendation is therefore semantic, not merely morphological: derive the form that names a real role, property, or manner, and keep an ordinary quantity as a bare cardinal.
Common Mistakes
Adding endings to a bare cardinal
❌ Mi vidis trijn novajn mesaĝojn.
Incorrect — the bare cardinal tri does not take plural or accusative endings.
✅ Mi vidis tri novajn mesaĝojn.
I saw three new messages.
Using an ordinal when quantity is intended
❌ Ni bezonas triajn volontulojn.
Incorrect for ‘three volunteers’ — triaj means ‘third’ in the plural.
✅ Ni bezonas tri volontulojn.
We need three volunteers.
Copying English month-first date order
❌ La limdato estas Junio dek-dua.
Incorrect as the neutral Esperanto date pattern.
✅ La limdato estas la dek-dua de Junio.
The deadline is June twelfth.
Leaving a measured extent unmarked
❌ La pakaĵo estas tri kilogramoj peza.
Incorrect in the bare measurement construction — the extent needs -n.
✅ La pakaĵo estas tri kilogramojn peza.
The package weighs three kilograms.
Treating a number noun like an invariant cardinal
❌ Ni mendis du dekduo da tasoj.
Incorrect — dekduo is a noun and must be plural after du.
✅ Ni mendis du dekduojn da tasoj.
We ordered two dozen cups.
Key Takeaways
Bare cardinals count and remain invariant. -O creates a noun that can take -j/-n and may govern da; direct -a and -e forms normally express ordinal position. Added suffixes create fractional, multiplicative, and grouping meanings. In practical notation, preserve the same logic: ordinal days and hours, cardinal quantities and labels, and case endings on the grammatical word that carries the relevant role.
Related Topics
- Cardinal NumbersA1 — Use Esperanto cardinal numbers to count, quantify noun phrases, identify items, and read basic dates, times, measurements, and decimal notation.
- Ordinal NumbersA1 — Form and use Esperanto ordinal numbers for sequence, labels, dates, clock hours, and ranked noun phrases.
- FractionsA2 — Build exact fractions with the official suffix -on-, then use their noun, adjective, and adverb forms with quantities and units.
- Multipliers and RepetitionsA2 — Distinguish multiplication with -obl- from repeated occasions with fojo, and form each pattern productively.
- Dates and YearsA2 — Form, inflect, write, and read Esperanto calendar dates and years without importing ambiguous English date order.
- Measures and PriceA2 — Mark bare measures of weight, size, value, and price with -n while distinguishing them from objects and prepositional phrases.