Breakdown of Мы обсуждали мрамор и холст, с которыми работал художник.
и
and
мы
we
работать
to work
художник
artist
Questions & Answers about Мы обсуждали мрамор и холст, с которыми работал художник.
Why is которыми in the plural here when both мрамор and холст are singular nouns?
Because the relative pronoun которыми refers back to both мрамор and холст combined. Since it represents multiple items, the pronoun must be plural. The instrumental plural form is которыми, paired with the preposition с.
Why do we need the preposition с and the Instrumental case for которыми?
The case of который is always determined by its role in its own part of the sentence (the relative clause). The verb работать uses the preposition с plus the Instrumental case to express what material someone is working with. So, 'worked with which' becomes с которыми работал.
Why is the word order работал художник instead of художник работал?
In Russian relative clauses, it is very natural to place the verb before the subject when the relative pronoun is not the subject. While с которыми художник работал is grammatically fine, putting the verb first makes the sentence flow much better.
What case are мрамор and холст in the first part of the sentence?
They are in the Accusative case because they are the direct objects of the verb обсуждали (we were discussing). Because they are inanimate masculine nouns, their Accusative forms look exactly the same as their basic Nominative forms.