Learn Russian: Мы начинаем поиск людей, имена которых мы раньше не знали. - We are starting a search for people whose names we did not know before.

When using который to mean "whose" (in the Genitive case), Russian places it directly after the noun it possesses. It helps to translate it literally in your head as "the names of whom".

How do we know to put которых in the plural Genitive form?

The word которых points back to людей (people), which is plural. We use the Genitive case to show possession, meaning we are talking about the names of these people.

Why is людей in the Genitive case here?

It follows the noun поиск (search). While English uses a preposition ("a search for people"), Russian simply connects the two nouns using the Genitive case, literally translating to "a search of people".

Is имена the regular plural of имя?

Yes! Имя (name) is a special neuter noun that ends in -мя. Its plural form is имена. Here it is in the Accusative case because it is the direct object of the verb, meaning it is the thing we did not know (не знали).