Relative Pronoun Který
Relative pronoun který forms a clause (a sentence-unit with a conjugated verb) that modifies a noun that precedes it, as in as the following example:
Look at the text which we read yesterday.
"which we read yesterday" further describes what type of text we are supposed to look at. It's not just a text, it's the text that we read yesterday.
Since který-clause is almost like an adjective in the sense that it modifies a specific noun, který behaves like an adjective in a way: it agrees in number and gender with the noun that it modifies; it declines like mladý. However, který must function within the ktery-clause itself. Který therefore must take its case in accord with its grammatical function within the ktery-clause.
Observe the following sentences and try to connect them with který:
(1a) Hledám profesora. 'I am looking for a professor'
(1b) Položila jsem mu otázku. 'I asked him a question.'
We want to turn these sentences into one: 'I am looking for the professor whom I asked a question.' There are three small steps to complete this task:
(1) The two sentences (1a, 1b) have one common entity: profesor. The first step to connect them with který is to determine the gender and number of profesor. Since it's masculine singular, we arrive at který, a masc. sg. form.
(2) The next step is to observe the pronoun that refers to the professor in (1b); it is an indirect object, and is in the dative case. The relative pronoun then must be kterému.
(3) The final step is to put kterému at the beginning of the relative clause preceded by a comma:
Hledám profesora, kterému jsem včera položila otázku.
The bookish counterpart for který is jenž.