Correlatives
The "correlatives" are a paradigm of pro-forms, used to ask and answer the questions what, where, when, why, who, whose, how, how much, and what kind. They are constructed from set elements so that correlatives with similar meanings have similar forms: There are nine endings corresponding to the nine wh- questions, and five initial elements that perform the functions of asking, answering, denying, being inclusive, and being indefinite about these nine questions. For example, the words kiam (when) and kiu (who, which), with the initial ki- of questions, ask about time and individuals, while the words tiam (then) and tiu (this/that one), with the same endings but the initial ti- of demonstratives, answer those questions, and the words neniam (never) and neniu (no-one) deny those questions. Thus by learning these 14 elements the speaker acquires a paradigm of 45 adverbs and pronouns.
The correlatives beginning with ti- correspond to the English demonstratives in th- (this, thus, then, there etc.), while ĉi- corresponds to every- and i- to some-. The correlatives beginning with ki- have a double function, as interrogative and relative pronouns and adverbs, just as the wh- words do in English: Kiu ĉevalo? (Which horse?), la ĉevalo, kiu forkuris (the horse which ran away).
The adjectival determiners ending in -u have the usual dual function of adjectives: standing alone as proforms, as in ĉiu (everyone); and modifying a noun, as in ĉiu tago (every day). Those ending in -io are exclusively used standing alone:ĉio (everything).
The correlatives have a genitive case ending in -es. Therefore the adjectival correlatives, ending in -ia and -iu, do not play that role, as adjectival personal pronouns such as mia "my" do. However, adjectival correlatives do agree in number and case with the nouns they modify, as any other adjectives: La ĉevaloj, kiujn mi vidis (The horses which I saw). They, as well as the independent determiners ending in -io, also take the accusative case when standing in for the object of a clause. The accusative of motion is used with the place correlatives in -ie, forming -ien (hither, whither, thither, etc.).
Read more about this topic: Esperanto Vocabulary