I have seen the usage of the word kio used as a question word "what" a lot of times:

Kio estas tio? (what's this?)

But lately I have seen this form:

Kion vi manĝas? (what are you eating?)

And I was confused. If "kio" can be used as an object of a sentence like the above example, can it be the object of a sentence which is not a question either (like in English)?

Tio estas kion mi vidis! (This is what I saw)

up vote 3 down vote accepted

Yes, you can use "kion" to introduce a relative sub-clause. Your sentence "Tio estas kion mi vidis!" is correct.

The interesting thing about this example sentence is that it touches a detail of Esperanto grammar that is not well documented in our most complete grammar resource PMEG. Let me explain.

A grammatically more complete way of saying "That is what I saw" would be "Tio estas tio, kion mi vidis." Now in constructions of the form "tio(n), kio(n)", one has the option to drop "tio(n)". PMEG says that normally dropping "tio(n)" is only permitted when it is in the same case as "kio(n)" (i.e. if both have the -n or both lack it).

However, PMEG also acknowledges that there are some contexts in which "tio" is commonly dropped from "tio, kion", but PMEG does not explain in detail in which contexts this is permissible. It only gives the example of sentences with "jen":

Jen [estas] tio, kion diris Aĥitofel! → Jen kion diris Aĥitofel!

By searching for the patterns "tio estas kion" and "tio estas tio kion" in the Tekstaro de Esperanto (an Esperanto corpus consisting of texts by well considered authors, inclusing many texts by Zamenhof), I realized that in such a context too, "tio" is commonly dropped despite being in a different case than the following "kion". Already Zamenhof himself dropped "tio" in such a context.

I will suggest to Bertilo Wennergren, the author of PMEG, to mention this context as another example, in which "tio" can be dropped before "kion", so as to make his explanation more complete (he has already previously made changes to PMEG based on my suggestions).

  • This answer is very interesting and informative but I think it is going into a lot of detail about a side issue (ie, whether the sentence needs the extra "tio") and doesn't respond much to the question in hand which I think is more about when to use "kio" and when to use "kion" to introduce the sub-phrase. – Neil Roberts Aug 25 '16 at 8:28
  • I wonder if 'tio' can be dropped here because 'esti' is intransitive, can never have a direct object, therefore 'kion' in the subphrase can never cause ambiguity. – jzknuckles Aug 25 '16 at 9:55

Yes, that is correct, you can use either kion or kio to introduce a sub-phrase. Which one to use depends on whether the thing it is referencing is being used as the subject or the object in the sub-phrase (not in the main phrase). For example:

Tio estas kion mi vidis!

(In this case the sub-phrase references the tio from the main phrase as the object)

Tio estas kio okazis

(In this case it is the subject)

Also note that using kio or kion is particular to this example and in some cases it is necessary to use kiu / kiun instead. Kio is only used in this case because it is referring to another correlative which also ends in o. In this case the correlative refers to tio so it must match and be kio. If the thing being referred to is a normal noun you would use kiu or kiun instead:

La pomo kiun mi manĝis estis bona.


La pomo kiu estis en la fridujo estis bona.

As always, there is a more detailed explanation of this in the grammar section of Lernu.

  • Note that you can only leave out the corresponding ti-correlative when the ki-correlative has the same case. Therefore, “Tio estas, kion mi vidis!” seems incorrect to me, unless you mean “There's that, which I saw!” (Which is usually written as “Estas tio, kion mi vidis!”) rather than “That is what I saw!”. For the latter your sentence should be “Tio estas tio, kion mi vidis!”. EDIT: See Marcos Cramer's answer. – Joffysloffy Aug 24 '16 at 20:14

[I now consider this old answer of mine incorrect. Please see my newer answer.]

To say "This is what I saw" in Esperanto, you have to say "Tio estas tio, kion mi vidis." In constructions of the form "tio(n), kio(n)", one has the option to drop "tio(n)", but only if it is in the same case as "kio(n)" (i.e. if both have the -n or both lack it). So you cannot drop "tio" in "tio, kion".

This is explained in some more detail and with some examples in PMEG.

  • Got it, thanks! – Yotam Salmon Aug 24 '16 at 20:56
  • Slight nitpicking: you wouldn't use "tio, kion...," but instead use "tio, kiun," because "that, what I ate" or something of the like doesn't work, but "that, which I ate" works just fine. – Clayton Ramsey Aug 25 '16 at 0:35
  • 1
    @ClaytonRamsey: I don't know of any reason to carry the English rule that "what" is not a relativizer over to Esperanto. – Max Aug 25 '16 at 6:15
  • After doing some more research on this issue, I realized that my above answer is not correct. I relied to heavily on PMEG, but your example sentence touches a grammatical detail that is not well documented in PMEG, as I have explained in detail in my new answer. I propose to delete this answer for this reason. – Marcos Cramer Aug 25 '16 at 8:20
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    @Clayton: The use of "kio" as a relative pronoun is absolutely standard in Esperanto. – Marcos Cramer Aug 25 '16 at 14:25

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