A short description of jenž
Jenž is a relative pronoun just like který. Just like a který-clause, a jenž-clause adds further information about a noun in the preceding clause. Thus, jenž-clause would appear in place of "about whom we have been discussing" in the following sentence:
This is not the portrait of Jára Cimrman about whom we are discussing.
The part in bold style further explains what type of person J. Cimrman is.
Just like který, jenž agrees in gender and number of the noun it refers back to, and takes its case from the jenž-clause.
Observe the following sentences and try to connect them with jenž:
(1a) To není portrét Járy Cimrmana. 'This is not a portrait of Jára Cimrman.'
(1b) O něm diskutujeme už řadu let. 'We have been discussing about [lit.] him for a number of years already.'
We want to turn these sentences into one: 'This is not a portrait of J. Cimrman about whom we have been discussing for a number of years alrady' Just like when we use který, there are three small steps to complete this task:
STEP 1: The two sentences (1a, 1b) have one common entity: Cimrman. The first step to connect them with jenž is to determine the gender and number of Cimrman. Since it's masculine singular, we arrive at jenž, a masc. sg. form. (see the table below)
STEP 2: Observe the pronoun that refers to Cimrman in (1b); it is part of the prepositional phrase taking the locative case (o + loc.). The relative pronoun then must be němž.
STEP : Put o němž at the beginning of the relative clause preceded by a comma:
To není portrét Járy Cimrmana, o němž mluvíme řadu let.