Kiu selects an individual: “who?” or “which one?” It can ask a question—Kiu venis?—or introduce a relative clause—la gasto, kiu venis. Unlike English “who,” kiu also selects things when the speaker is choosing among identifiable individuals: Kiu libro estas via?
Its form belongs to Esperanto's closed table-word system. Initial ki- supplies question or relative force; final -u supplies the domain “individual person or thing.” The allowed endings -j and -n then show plural and accusative roles: kiu, kiuj, kiun, kiujn.
Kiu inside the 5×9 system
The 45 correlatives combine five initial series with nine final meanings. These elements are stable; do not mix them with ordinary endings to invent new hybrids.
| Dimension | Forms relevant to kiu |
|---|---|
| Five initial series | ki- question/relative, ti- demonstrative, i- indefinite, ĉi- universal, neni- negative |
| Nine final domains | -u individual, -o thing, -a kind, -es possessor, -e place, -am time, -al reason, -el manner, -om quantity |
| The -u column | kiu, tiu, iu, ĉiu, neniu |
| The ki- row | kiu, kio, kia, kies, kie, kiam, kial, kiel, kiom |
Thus ki-u asks or relates identity, while ki-o asks “what thing,” ki-a asks “what kind,” and ki-e asks “where.” This architecture helps you select both dimensions instead of translating an English wh-word mechanically.
For the complete grid, see The Table-Word System.
Kiu in direct questions
Use kiu when asking which person or individual item fills a role.
Kiu venos al la kunveno hodiaŭ?
Who will come to the meeting today?
Kiu buso iras al la flughaveno?
Which bus goes to the airport?
— Kiu estas via kuracisto? — Doktoro Marin.
— Who is your doctor? — Doctor Marin.
When kiu directly precedes a noun, it functions as a determiner. Do not add la: kiu buso, not la kiu buso. When the noun is obvious, leave it understood: Kiu estas via? can mean “Which one is yours?”
KI-words normally stand at the beginning of their clause even when their grammatical role is object or complement.
Number and accusative case
The -u correlatives are among the table words that accept both plural -j and accusative -n. Put -j before -n.
| Form | Role | Typical question |
|---|---|---|
| kiu | one individual, non-accusative | Kiu venis? |
| kiuj | several individuals, non-accusative | Kiuj venis? |
| kiun | one individual, accusative | Kiun vi vidis? |
| kiujn | several individuals, accusative | Kiujn vi vidis? |
Kiuj kolegoj partoprenos la kurson?
Which colleagues will take part in the course?
Kiun libron vi elektis por la vojaĝo?
Which book did you choose for the trip?
Kiujn fotojn ni sendu al la familio?
Which photos should we send to the family?
When a noun is present, both words agree: kiuj kolegoj, kiun libron, kiujn fotojn. When the noun is omitted, kiu keeps the endings needed for the understood noun.
After an ordinary preposition, use the base form required by number, without object -n:
Kun kiu vi venis al la festo?
Who did you come to the party with?
Al kiuj ni sendu la konfirmon?
To whom should we send the confirmation?
Kiu in embedded questions
The same question word introduces an indirect question. Its case still comes from its role inside that question.
Mi ne scias, kiu havas la rezervan ŝlosilon.
I do not know who has the spare key.
Ĉu vi memoras, kiun ni renkontis ĉe la teatro?
Do you remember whom we met at the theater?
In the first, kiu is subject of havas. In the second, kiun is object of renkontis. The main verbs scias and memoras do not determine its case.
Relative kiu
Relative kiu represents a previously mentioned person or individual thing inside a describing clause. It agrees in number with that antecedent, but its case is determined by its job inside the relative clause.
La virino, kiu salutis min, laboras ĉe la banko.
The woman who greeted me works at the bank.
La virino, kiun mi salutis, laboras ĉe la banko.
The woman whom I greeted works at the bank.
The antecedent is singular la virino in both sentences. In the first relative clause, kiu is the greeter and therefore the subject. In the second, kiun is the person greeted and therefore the direct object.
La dokumentoj, kiuj alvenis hodiaŭ, estas sur via tablo.
The documents that arrived today are on your desk.
La dokumentoj, kiujn vi sendis hieraŭ, jam alvenis.
The documents that you sent yesterday have already arrived.
Plural antecedent dokumentoj requires kiuj/kiujn. Again, only the second relative pronoun takes -n, because only the second is an object inside its clause.
A preposition may introduce the relative pronoun:
Jen la kolego, kun kiu mi laboras pri la raporto.
Here is the colleague with whom I am working on the report.
English often ends with the preposition (“the colleague I work with”); Esperanto places kun before kiu.
Kiu, kio, and kia
English “what/which” does not map to one Esperanto word. Choose the semantic domain.
Kiu taso estas via?
Which cup is yours?
Kia taso estas plej taŭga por teo?
What kind of cup is most suitable for tea?
Kio falis en la kuirejo?
What fell in the kitchen?
Kiu taso selects an identifiable individual cup. Kia taso asks for a type or quality. Kio asks what thing or event is involved without supplying a noun. Although bare kiu defaults to a person when context supplies no other class, it can represent an understood object: Jen tri tasoj; kiun vi volas?
— Jen tri liberaj seĝoj; kiun vi volas? — Tiun apud la fenestro.
— Here are three free chairs; which one do you want? — The one by the window.
Free relative use
Kiu can introduce a general relative without an expressed antecedent, with an understood tiu: Kiu frue venas, trovas bonan sidlokon means “Whoever comes early finds a good seat.” This is neutral proverbial-style usage, not a separate form.
Kiu bezonas helpon, povas telefoni al ĉi tiu numero.
Anyone who needs help can call this number.
Common Mistakes
1. Using kio before a person noun
❌ Kio persono telefonis al vi?
Incorrect: a person is an individual, so use the -u correlative.
✅ Kiu persono telefonis al vi?
Which person called you?
2. Forgetting accusative case
❌ Kiu vi invitis al la festo?
Incorrect: the questioned person is the direct object of invitis.
✅ Kiun vi invitis al la festo?
Whom did you invite to the party?
3. Adding accusative to a subject
❌ Kiujn venis hieraŭ?
Incorrect: the questioned people are the subject of venis.
✅ Kiuj venis hieraŭ?
Who came yesterday?
4. Combining kiu with la
❌ La kiu buso iras al la centro?
Incorrect: kiu itself determines the noun.
✅ Kiu buso iras al la centro?
Which bus goes downtown?
5. Copying the antecedent's case into the relative clause
❌ Mi legis la libron, kiun kuŝis sur la tablo.
Incorrect: although libron is an object in the main clause, the relative pronoun is subject of kuŝis.
✅ Mi legis la libron, kiu kuŝis sur la tablo.
I read the book that was lying on the table.
Key Takeaways
Kiu combines question/relative ki- with individual -u. It selects people or identifiable individual things, either alone or before a noun. Use kiu/kiuj for non-accusative roles and kiun/kiujn for accusative roles; after an ordinary preposition, do not add object -n. Relative kiu matches its antecedent in number, but its case comes entirely from its role inside the relative clause.
Now practice Esperanto
Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.
Start learning Esperanto →Related Topics
- Kio: The Question/Relative Thing CorrelativeA1 — Use kio to ask about an unspecified thing or idea and to link relative clauses to thing-like or clause-like antecedents.
- Kia: The Question/Relative Kind Or Quality CorrelativeA1 — Use kia to ask what kind or what quality, with adjective-like agreement and precise contrasts with kiu, kio, and kiel.
- The Table-Word SystemA1 — Learn Esperanto's 45 table words as a five-by-nine meaning system for questions, demonstrations, indefinites, universals, and negatives.
- Case and Number on CorrelativesA2 — Learn exactly which correlative families accept plural -j, object -n, or directional -n, and which remain invariant.
- Relative Clauses with KiuA2 — How kiu, kiuj, kiun, and kiujn introduce finite clauses that identify or describe people and things.
- Relative Case inside the ClauseA2 — How the case and preposition of kiu are determined inside the relative clause, independently of the antecedent’s role.