kdo / co

These pronouns stand alone and are used across all levels of formality.

1. Strict Verb Agreement
Because you don't know the answer to your question yet, Czech assigns a fixed grammatical gender and number to these pronouns. Kdo is strictly masculine singular (which dictates the past tense verb ending in -l) and declines as an animate noun. Co is strictly neuter singular (past tense ending in -lo). When used with action verbs, these pronouns always take a singular verb, even if you are asking about multiple people or things:
• Kdo to udělal? (Who did that?)
• Co se stalo? (What happened?)
However, there is an exception: when using the verb být (to be) to link to a plural noun, the verb agrees with that noun and becomes plural (e.g., Kdo jsou ti lidé? – Who are those people?).

2. Word Order and Clitics
Because kdo and co are question words, they usually sit at the very beginning of a sentence. This means that any clitics (short words like the past tense auxiliary jsi or the reflexive pronoun se) must immediately follow them in the second position: Co jsi dělal? (What did you do?) / Kdo se ptal? (Who asked?). If a preposition is used, the preposition and the pronoun count together as a single block in the first position, so the clitic follows the pronoun: S kým jsi mluvil? (Who did you speak with?).

3. Changing Forms (Cases) and Prepositions
Like nouns, kdo and co change their endings depending on their role in the sentence or the preposition used. Because kdo is treated as masculine animate, its accusative and genitive forms are identical (koho). Because co acts as a neuter concept, its accusative form is identical to its nominative form (co)—a core rule for all neuter and masculine inanimate words in Czech. The stem for co changes to č- in other cases. The forms are highly irregular: genitive (koho, čeho), dative (komu, čemu), locative (e.g., o kom, o čem—note that the preposition o is included here because the locative case never exists without a preposition in Czech), and instrumental (e.g., kým, čím). Additionally, when co is used with certain prepositions in the accusative, it can contract into a single word (e.g., na co → nač, za co → zač, o co → oč). Note that these contracted forms lean formal, literary, or are used in fixed phrases (like Oč jde? – What is it about? or the very common phrase Není zač – You're welcome). Fun fact: the standard Czech word for "why" (proč) is historically this exact same type of contraction (pro co)! In everyday casual speech, people usually prefer the uncontracted forms (na co, za co, o co). You can learn more about this in Case forms of kdo and co.

Important Note on Prepositions: Unlike in English, where prepositions can appear at the end of a sentence (e.g., "Who are you going with?"), in Czech, the preposition must always come right before the pronoun: S kým jdeš? (With whom are you going?).

4. Co vs. Jaký
English uses "what" for many different concepts, but Czech is more specific. Use co to ask for the basic identity of a noun (a bare thing). If you want to ask "what kind of" or "what is it like" (asking about qualities), you must use the word jaký instead.

5. Connecting Clauses and Colloquial Uses
Besides asking questions, kdo and co act as relative pronouns to connect clauses. They are usually paired with demonstrative pronouns: Ten, kdo to udělal... (The one who did it...) or To, co chci... (That which I want...). You can learn more about this in Free relative kdo and co. You will also often hear the forms kdopak and copak. While these can be emphatic ("who exactly" and "what exactly"), adding the -pak suffix is extremely common in spoken Czech to soften a question, make it sound friendlier, or express curiosity. In colloquial spoken Czech, co is frequently used to mean "why" (e.g., Co se směješ? – Why are you laughing?). Additionally, co is widely used in casual speech as a universal relative pronoun replacing který (e.g., Ten kluk, co tam stál – The boy who stood there).

6. Building New Words
Kdo and co serve as the root for many other useful pronouns. By adding prefixes, you can easily expand your vocabulary to include words like někdo (someone), něco (something), nikdo (no one), and nic (nothing).