Replacing relative clauses with compact, literary participial constructions.
A participial phrase (причастный оборот) is a compact way of attaching a description to a noun without using a full relative clause introduced by который ("which/who"). It's built from a participle — a verb form that behaves like an adjective — plus any words that depend on it.
Compare these two equivalent sentences:
Студент, который читает книгу, сидит у окна. (The student who is reading a book is sitting by the window.) — relative clause, everyday register.
Студент, читающий книгу, сидит у окна. (The student reading a book is sitting by the window.) — participial phrase, more compact and literary.
Both are grammatically correct and mean the same thing. The participial version simply replaces "который читает" with the single participle читающий (reading), which itself declines like an adjective to agree with студент in gender, number and case.
A participial phrase is set off with commas only when it follows the noun it describes — exactly as shown above. When the participial phrase comes before the noun, no comma is needed: Читающий книгу студент сидит у окна. This comma rule is tested frequently in Russian school exams and in TORFL writing assessments, so it's worth memorising as a fixed rule rather than relying on instinct.
Passive past participles are by far the most frequent in everyday written Russian — newspaper headlines, official notices and instructions rely on them constantly (e.g. закрытый магазин, "a closed shop").
Participial phrases are a hallmark of written, formal, and literary Russian — news articles, academic texts, official documents, and 19th-century literature use them extensively. In spoken conversation, Russians overwhelmingly prefer the который-clause instead, so treat participial phrases primarily as a reading-comprehension and formal-writing skill rather than something to force into casual speech.