Learn Russian: Пергамент, купленный монахом, был отличного качества. - The parchment, bought by the monk, was of excellent quality.

This touches on the core rule for participial phrases in Russian. Because the phrase купленный монахом comes immediately after the noun it modifies (Пергамент), it must be separated by commas. If we moved the phrase before the noun (Купленный монахом пергамент был...), no commas would be used.

Why does the participle купленный take the -ый ending?

A participle acts like an adjective, meaning it has to agree in gender, number, and case with the noun it describes. Since Пергамент is a masculine, singular noun in the nominative case (it is the subject of the sentence), купленный takes the matching masculine nominative ending.

Why is the word for monk (монах) in the instrumental case here as монахом?

Купленный is a passive participle, meaning 'bought'. In Russian passive constructions, the agent (the person or thing actually doing the action) is always expressed in the instrumental case. Therefore, 'by the monk' translates to монахом.

Why does the sentence use the genitive case for отличного качества?

This is a common structure called the Genitive of Quality. When describing an object's specific trait (like its quality, size, or color) using a noun-adjective pair, Russian uses the genitive case. It literally translates to 'was of excellent quality', just like in English.