Learn Russian: Холст, с которым художник пошёл в парк, был совершенно пустой. - The canvas, with which the painter went to the park, was completely empty.

The word который must match the gender and number of the noun it replaces, which is the masculine noun холст. However, its case depends on its role in its own part of the sentence. Because the artist went to the park with the canvas, we need the preposition с (with) followed by the Instrumental case. The masculine Instrumental form of который is которым.

Why are there commas around the phrase с которым художник пошёл в парк?

In Russian, you must always place commas around a relative clause (a descriptive phrase starting with a word like который). Unlike in English, where we sometimes leave these commas out, Russian punctuation strictly requires them to separate the extra information from the main sentence.

Why does this sentence use пошёл rather than another verb of motion like шёл or ходил?

пошёл is the perfective verb meaning to set off or depart on foot. The sentence focuses on the single, completed action of the artist leaving for the park with the canvas. If we used шёл, it would mean he was in the process of walking, and ходил would imply he made a round trip or went there regularly.

Why is it в парк (Accusative case) instead of в парке (Prepositional case)?