Breakdown of У нас есть семейный архив, из которого я взял этот документ.
я
I
мы
we
этот
this [f: эта, n: это]
у
at, by
Questions & Answers about У нас есть семейный архив, из которого я взял этот документ.
Why is the word которого in the Genitive case?
The case of который depends on its role in its own part of the sentence. Here, it follows the preposition из (from/out of), which always requires the Genitive case. So, из которого means "from which".
How do we know to use the masculine form которого instead of a feminine or plural form?
If "this document" is the direct object being taken, why don't the words этот документ change their endings?
They are actually in the Accusative case here! However, because документ is an inanimate masculine noun (a non-living thing), its Accusative form looks exactly like its Nominative (dictionary) form. This rule also applies to the modifier этот.
Why use the perfective verb взял here instead of the imperfective брал?
The perfective verb взял is used because taking the document was a single, successfully completed action. The speaker took it once, and as a result, they now have it. The imperfective брал would sound like a repeated action, such as "I used to take".