kdo / co (v pádech)

The interrogative pronouns kdo and co are essential for asking questions about identity. Just like nouns, they must be declined into different cases depending on the verb or preposition they are used with.

1. Declension of Kdo (Who)
Used primarily for people. (Note: While co is the standard question for identifying an animal, native speakers may use kdo for individualized animals or pets, treating them as animate beings.) Its declension follows the pronominal declension (similar to the pronoun ten). Notice that the Genitive and Accusative forms are identical:

• Nominative: kdo (Who?)
• Genitive: koho (od koho) (Of whom? / From whom?)
• Dative: komu (ke komu) (To whom?)
• Accusative: koho (Whom?)
• Locative: kom (o kom) (About whom? - always used with a preposition)
• Instrumental: kým (s kým) (With whom? / By whom?)

Note: To ask "Whose?", do not use the genitive koho. Instead, use the possessive interrogative pronoun čí, which declines like a soft adjective.

2. Declension of Co (What)
Used for things, abstract concepts, and animals (e.g., Co je to? To je pes. - What is it? It's a dog.).

Notice that for co, the Nominative and Accusative forms are identical, which is a general rule for all neuter words (nouns, adjectives, pronouns) and most inanimate masculine words in Czech (with exceptions like personal and relative pronouns). In all cases other than the Nominative and Accusative, the stem changes to a soft č-:

• Nominative: co (What?)
• Genitive: čeho (od čeho) (Of what? / From what?)
• Dative: čemu (k čemu) (To what?)
• Accusative: co (What?)
• Locative: čem (o čem) (About what? - always used with a preposition)
• Instrumental: čím (s čím) (With what? / By what?)

Note: The Locative case in Czech can never be used without a preposition (such as v, na, o, po, při). Other cases can also be used with various prepositions (e.g., bez koho, pro co). Additionally, when co is used in the Accusative case with prepositions like na, o, and za, it frequently merges with them to form nač, oč, and zač in written or slightly formal Czech (this does not apply to the Locative case, so o čem cannot be contracted). Crucially, the common word proč (why) is actually a merger of pro + co. In informal spoken Czech (obecná čeština), the long vowels in the Instrumental case are frequently shortened, so kým becomes kym and čím becomes čim.

Crucial note for English speakers: In Czech, prepositions can never be left at the end of a sentence. They must always immediately precede the declined pronoun. For example, you must say S kým jdeš? (With whom are you going?) and never Kým jdeš s?

3. Verb Agreement
These pronouns have no plural forms and have a fixed inherent grammatical gender. When they are the subject of an action, they force fixed singular verb agreement, regardless of the expected answer. In the past tense, this means kdo always takes the masculine animate singular ending -l (e.g., Kdo tam byl? - Who was there?), and co always takes the neuter singular ending -lo (e.g., Co se stalo? - What happened?). However, when using the verb být (to be) to identify a specific noun, the verb agrees with that noun instead (e.g., Kdo byla ta žena? - Who was that woman?, Kdo bylo to dítě? - Who was that child?).

4. Derived Pronouns
By adding prefixes like ně- or ni-, you create indefinite and negative pronouns (někdo, něco, nikdo, nic). Někdo and nikdo decline using the exact same endings as kdo. For něco and nic, the oblique cases use the exact same endings attached to a něč- or nič- stem (e.g., něčeho, něčemu). However, note that the Nominative and Accusative form is nic (not "nico"). Crucially, negative pronouns require a negative verb in Czech (double negation), such as Nikdo to neví (Nobody knows it) or Nic nevidím (I see nothing). When using prepositions with negative pronouns, the preposition is placed before the entire word (e.g., s nikým - with no one, o ničem - about nothing).

5. Relative Pronouns
Besides asking questions, kdo and co are equally important as relative pronouns used to connect clauses. For example: To je to, co chci. (That is what I want.) or Ten, kdo to udělal... (The one who did it...). Additionally, in colloquial spoken Czech (obecná čeština), co is frequently used as a universal relative pronoun replacing který (e.g., Ten chlap, co tam stál... - The guy who was standing there...).